Ringing preventive circuit for removing noise in an electronics transmission on path of semiconductor testing apparatus

ABSTRACT

To remove ringing from pulse. A buffer amplifier (30) is inserted in series into a path through which a signal is transmitted. Ringing caused in the output of the buffer amplifier (30) is detected by an L side comparator (31) and an H side comparator (32). When ringing is detected, a current is supplied into the input of the buffer amplifier (30) through an L side current supply circuit (33) or an H side current supply circuit (34). By supplying a current from the L side and H side current supply circuits (33, 34), the overshooting portion due to ringing can be suppressed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to removal of ringing in the input unit ofa pin electronics card used in semiconductor testing apparatus, outputstage circuit inside the IC or LSI, or output unit of a device undertest board (DUT board) used in semiconductor testing apparatus, and moreparticularly to a ringing preventive circuit for removing noise such asringing from a transmission path in which an electric signal istransmitted.

2. Description of the Background Art

FIG. 25 is a conceptual diagram showing the connected state of the DUTboard mounting a semiconductor device such as IC and LSI, and the inputunit of a pin electronics card of a semiconductor testing apparatus, fortesting a semiconductor apparatus such as IC and LSI. In FIG. 25,reference numeral 1 denotes an objective device to be tested such as ICand LSI, 2 is a DUT board for mounting the objective device 1, 3 is aholder such as socket and prober fixed on the DUT board 2 for holdingand electrically connecting the objective device 1, 4 is a transmissionpath for transmitting an electric signal outputted from the output unitof the objective device 1, being connected to the objective device 1 inthe holder 3, 5 is a connector connected to the transmission path 4provided at the DUT board 2 side, 6 is a semiconductor testing apparatusfor testing the objective device 1,7 is a pin electronics card providedin the semiconductor testing apparatus 6 and including a comparator orthe like, and 8 is a connector provided in a pin electronics card 7 tobe connected to the connector 5 of the DUT board 2.

The pin electronics card 7 of the semiconductor testing apparatus 6 isan input and output interface with the objective device 1, and has asignificant influence on the test performance.

FIG. 26 is a circuit diagram showing the outline of the structure of thepin electronics card 7 of the semiconductor testing apparatus shown inFIG. 25. In FIG. 26, reference numeral K1 is a relay having one endconnected to the connector 8 for making or breaking a connection betweenthe one end and the other end in response to a control signal, 11 is apin electronics comparator having an input terminal 11a connected to theother end of the relay K1, a terminal connected to a power source E5 tobe provided with a reference voltage V_(Hth), a terminal connected to apower source E6 to be provided with a reference voltage V_(Lth), and anoutput terminal 11b for outputting the result of comparison of thevoltage at the input terminal 11a with the reference voltages V_(Hth),V_(Lth), 12 is a pin electronics driver having an input terminal 12a forreceiving a signal to be given to the objective device for testing, andan output terminal 12b connected to the other end of the relay K1, and13 is a pin electronics control circuit for processing the signal to begiven to the objective device from the semiconductor testing apparatusto give to the input terminal 12a of the pin electronics driver 12,processing the signal outputted from the output terminal 11b of the pinelectronics comparator 11 to be taken into the semiconductor testingapparatus, and controlling to open or close the relay K1. Herein, it issupposed that reference voltage V_(Hth) >reference voltage V_(Lth).

The operation of the conventional semiconductor testing apparatus shownin FIG. 25 and FIG. 26 is described below. In particular, processing ofthe signal to be outputted from the objective device 1 is explained. Inthe pin electronics card 7, in order to process an input signal, therelay K1 is in ON (closed) state, so that the pin electronics comparator11 may be connected between the connector 8 and pin electronics controlcircuit 13. The voltage V_(CO) at the output terminal 11b of the pinelectronics comparator 11 is, supposing the voltage at the inputterminal 11a of the pin electronics comparator 11 to be V_(CI), V_(CO)=V_(CL) when V_(Ci) >V_(Hth), V_(CO) =V_(CL) when V_(Ci) <V_(Lth), andV_(CO) =V_(CH) when V_(Lth), <V_(Ci) <V_(Hth), where voltage V_(CL) isthe lower side logic level in the pin electronic control circuit 13, andvoltage V_(CH) is the higher side logic level in the pin electronicscontrol circuit 13.

FIG. 27 is a waveform diagram showing the signal processed by the pinelectronics card and the processing result in the pin electronicscomparator. The waveform shown in (a) of FIG. 27 is a voltage waveformof an ideal electric signal suited to processing of the pin electronicscomparator 11. Ideally, from the output circuit of the objective device1 shown in FIG. 25, a signal having a waveform as shown in (a) of FIG.27 is outputted, and passes through the transmission path 4, and asignal having the waveform shown in (a) of FIG. 27 is put into theconnector 8 of the pin electronics card 7 in the semiconductor testingapparatus 6. From the connector 8 which is the input and output terminalof the pin electronics card, an ideal signal Vi is put in, and it issupplied into the pin electronics comparator 11 through the relay K1.Therefore, the voltage V_(CI) at the input terminal 11a of the pinelectronics comparator is ideally a signal Vi having the waveform shownin (a) of FIG. 27. At this time, the output impedance of the pinelectronics driver 12 is supposed to be in high impedance state(hereinafter called Hi-Z state). Hence, ideally, the voltage waveformgenerated at the output terminal 11b by the action of the pinelectronics comparator 11 is as shown in (b) of FIG. 27.

Actually, however, in the process of the waveform passing through thetransmission path 4 and others, the signal having the voltage waveformas shown in FIG. 28 mixed with ringing and other noise due to inductancecomponent and capacitance component of the transmission path 4,impedance mismatching of the transmission path 4, incoming noise mixedin the transmission path 4 and others is put into the pin electronicscomparator 11.

FIG. 28 is a waveform diagram showing the signal processed in the pinelectronics card and the processing result in the pin electronicscomparator. When a signal having the waveform as shown in (a) of FIG. 28is put into the pin electronics comparator 11, the pin electronicscomparator 11 may sometimes malfunction. When the waveform shown in (a)of FIG. 28 is put into the pin electronics comparator 11, malfunctionoccurs in the portion crossing the voltage V_(Hth) in shaded region B in(a) of FIG. 28, and in the portion crossing the voltage V_(Lth) inshaded region D. When the signal having the waveform shown in (a) ofFIG. 28 is put into the pin electronics comparator 11, the waveformappearing at the output terminal 11b of the pin electronics comparator11 is as shown in (b) of FIG. 28. Shaded region E in (b) of FIG. 28 isthe malfunctioning portion.

The pin electronics card is an interface between the objective device 1and semiconductor testing apparatus 6, and the information indicated bythe signal outputted from the objective device 1 must be accuratelycommunicated to the semiconductor testing apparatus 6, but this objectmay not be achieved if malfunction occurs in the pin electronicscomparator 11.

Incidentally, transmission of pulse signal or processing is not limitedto the semiconductor testing apparatus alone, but such scene is oftenencountered in general semiconductor devices. FIG. 29 is a conceptualdiagram showing part of the constitution of a general semiconductordevice. In FIG. 29, reference numeral 14 is a semiconductor device, 15is an output pin of the semiconductor device 14, 16 is an output bufferconnected to the output pin 15 for giving an output signal to the outputpin 15, 17 is a transmission path connected to the output buffer 16, and18 is a circuit before an output stage for generating a circuit to betransmitted through the transmission path 17. The transmission path 17may contain wiring of aluminum, polysilicon or the like, in thesemiconductor device 14, and the output pin 15 may be a pad or the like.

A pulse signal having a certain waveform is outputted from the circuitbefore an output stage 18, and is transmitted to the output buffer 16through the transmission path 17. It is further transmitted from theoutput buffer 16 having a low output impedance to the output pin 15, andis outputted outside the semiconductor device from the output pin 15.The waveform of the signal outputted from the circuit before an outputstage 18 and the waveform transmitted to the output pin 15 should beideally ringing-free waveform as shown in (a) of FIG. 27.

However, for example, when a signal having a waveform as shown in (a) ofFIG. 27 is outputted from the circuit before an output stage 18, in theprocess of passing through the transmission path 17 or output buffer 16,it may be changed to a waveform having noise such as ringing, as shownin (a) of FIG. 28, due to inductance component in the transmission path17, crosstalk noise from the peripheral circuit or the like, and may betransmitted to the output pin 15 in such form.

The conventional semiconductor testing apparatus, DUT board, andsemiconductor device are thus constituted, and ringing or other noise iscaused in the process of the electric signal passing through thetransmission paths 4, 17, and therefore when the signal mixed withringing is processed, the circuit and others may malfunction, andaccurate testing of semiconductor device or appropriate action ofsemiconductor device may not be realized.

SUMMARY OF THE INVENTION

A first aspect of the invention relates to a ringing preventive circuitcomprising a buffer amplifier having an input terminal and an outputterminal, a first comparator having a first input terminal connected tothe output terminal of the buffer amplifier, a second input terminalprovided with a first reference voltage, and an output terminal, foroutputting a first control signal from the output terminal in responseto a result of comparison between the voltages at the first and secondinput terminals, and a first current supply circuit having a firstterminal connected to a first power source for supplying a firstvoltage, a second terminal connected to the input terminal of the bufferamplifier, and a control signal input terminal connected to the outputterminal of first comparator, for controlling conduction andnon-conduction of a current flowing from the first terminal to thesecond terminal by the first control signal.

A second aspect of the invention relates to a ringing preventive circuitof the first aspect, wherein the first current supply circuit comprisesa constant current source having a first terminal and a second terminalconnected to the first power source, and a second terminal, forsupplying a specific current from the first terminal to the secondterminal, and switch means having a first current electrode connected tothe first terminal of the constant current source, a second currentelectrode connected to the input terminal of the buffer amplifier, and acontrol electrode connected to the second terminal of the firstcomparator, for controlling on/off of the current flowing between thefirst and second current electrodes in response to the signal given tothe control electrode.

A third aspect of the invention relates to a ringing preventive circuitof the first aspect, wherein the first current supply circuit comprisesswitch means having a control electrode connected to the output terminalof the first comparator, a first current electrode provided with asecond voltage relating to the first power source, and a second currentelectrode, for controlling conduction and non-conduction of a currentflowing between the first and second current electrodes in response tothe signal given to the control electrode, and a current mirror circuitconnected to the first power source, having a first current outputterminal connected to the second current electrode of the switch means,and a second current output terminal connected to the input terminal ofthe buffer amplifier, for outputting a first current from the firstcurrent output terminal and a second current from the second currentoutput terminal, the second current in the second current outputterminal is duplicated in said first current output terminal.

A fourth aspect of the invention relates to a ringing preventive circuitof the first aspect, further comprising a second comparator having afirst input terminal connected to the output terminal of the bufferamplifier, a second input terminal provided with a second referencevoltage, and an output terminal, for outputting a second control signalfrom the output terminal in response to a result of comparison betweenvoltages of the first and second input terminals of the secondcomparator, and a second current supply circuit having a first terminalconnected to a second power source for supplying a second voltage, asecond terminal connected to the input terminal of the buffer amplifier,and a control signal input terminal connected to the output terminal ofthe buffer amplifier, for controlling conduction and non-conduction of acurrent flowing from the first terminal to the second terminal inresponse to the second control signal.

A fifth aspect of the invention relates to a ringing preventive circuitcomprising a buffer amplifier having an input terminal and an outputterminal, a first comparator having a first input terminal connected tothe output terminal of the buffer amplifier, a second input terminalprovided with a first reference voltage, and an output terminal, foroutputting a first control signal from the output terminal in responseto a result of comparison between voltages at the first an second inputterminals, and a first current supply circuit having a first terminalconnected to a first power source for supplying a first voltage, asecond terminal connected to the input terminal of the buffer amplifier,a third terminal connected to the output terminal of the bufferamplifier, and a control signal input terminal connected to the outputterminal of first comparator, for controlling conduction andnon-conduction by the first control signal, and passing a currentdepending on the voltage of the third terminal from the first terminalto the second terminal.

A sixth aspect of the invention relates to a ringing preventive circuitof the fifth aspect, further comprising a second comparator having afirst input terminal connected to the output terminal of the bufferamplifier, a second input terminal provided with a second referencevoltage, and an output terminal, for outputting a control signal fromthe output terminal, in response to a result of comparison betweenvoltages of the first and second input terminals, and a second currentsupply circuit having a first terminal connected to a second powersource for supplying a second voltage, a second terminal connected tothe input terminal of the buffer amplifier, a third terminal connectedto the output terminal of the buffer amplifier, and a control signalinput terminal connected to the output terminal of second comparator,for controlling conduction and non-conduction by the second controlsignal, and passing a current depending on the voltage of the thirdterminal from the first terminal to the second terminal.

A seventh aspect of the invention relates to a ringing preventivecircuit of the fifth aspect, wherein the first current supply circuitcomprises a differential amplifying circuit having a first power sourceterminal provided with a second voltage relating to the first powersource, a first input terminal provided with a second reference voltage,a second input terminal connected to the output terminal of the bufferamplifier, and an output terminal, for amplifying the voltage differencebetween the input terminal and the second input terminal, and outputtingon the basis of the second voltage, a transistor having a first currentelectrode connected to the output terminal of the differential amplifiercircuit, a control electrode connected to the output terminal of thefirst comparator, and a second current electrode, and a current mirrorcircuit having a first current output terminal connected to the secondcurrent electrode of the transistor, a second current output terminalconnected to the input terminal of the buffer amplifier, and a currentinput terminal connected to the first power source, for outputting afirst current from the first current output terminal and a secondcurrent from the second current output terminal, the second current inthe second current output terminal is duplicated in the first currentoutput terminal.

An eighth aspect of the invention relates to a device under test boardcomprising a connector for connecting with a test head, a transmissionpath for transmitting a signal to the connector, holding means forholding an objective device and electrically connecting the transmissionpath and the objective device, and a ringing preventive circuitconnected between the transmission path and the connector, for removingringing of the signal transmitted through the transmission path, whereinthe ringing preventive circuit comprises a buffer amplifier having aninput terminal connected to the transmission path and an output terminalconnected to the connector, a first comparator having a first inputterminal connected to the output terminal of the buffer amplifier, asecond input terminal provided with first reference voltage, and anoutput terminal, for outputting a control signal from the outputterminal in response to a result of comparison between voltages of thefirst and second input terminals, and a first current supply circuithaving a first terminal connected to a power source for supplying asupply voltage, a second terminal connected to the input terminal of thebuffer amplifier, and a control signal input terminal connected to theoutput terminal of the first comparator, for controlling conduction andnon-conduction of a current flowing from the first terminal to thesecond terminal in response to control signal.

A ninth aspect of the invention relates to a device under test boardcomprising a connector for connecting with a test head, a transmissionpath for transmitting a signal to the connector, holding means forholding an objective device and electrically connecting the transmissionpath and the objective device, and a ringing preventive circuitconnected between the transmission path and the connector, for removingringing of the signal transmitted through the transmission path, whereinthe ringing preventive circuit comprises a buffer amplifier having aninput terminal connected to the transmission path and an output terminalconnected to the connector, a first comparator having a first inputterminal connected to the output terminal of the buffer amplifier, asecond input terminal provided with first reference voltage, and anoutput terminal, for outputting a control signal from the outputterminal in response to a result of comparison between voltages of thefirst and second input terminals, and a first current supply circuithaving a first terminal connected to a power source for supplying asupply voltage, a second terminal connected to the input terminal of thebuffer amplifier, a third terminal connected to the output terminal ofthe buffer amplifier, and a control input terminal connected to thesecond output of the first comparator, for controlling on/off by thecontrol signal, and passing a current depending on the voltage of theoutput terminal of the buffer amplifier from the first terminal to thesecond terminal.

A tenth aspect of the invention relates to a device under test board ofthe ninth aspect, further comprising a high impedance circuit insertedbetween the input terminal of the buffer amplifier and the transmissionpath, for setting the impedance of the transmission path as seen fromthe buffer amplifier higher than the impedance of the transmission path.

An eleventh aspect of the invention relates to a pin electronics cardcomprising a connector connected to a test board on which an objectivedevice is mounted, an interface circuit having a signal input terminalfor interfacing to use a signal entered from the signal input terminalas an input to the tester, and a ringing preventive circuit connectedbetween the signal input terminal of the interface circuit and theconnector, for removing ringing caused in a signal entered from thesignal input terminal, wherein the ringing preventive circuit comprisesa buffer amplifier having an input terminal connected to the connector,and an output terminal connected to the signal input terminal of theinterface circuit, a first comparator having a first input terminalconnected to the output terminal of the buffer amplifier, a second inputterminal provided with a first reference voltage, and an outputterminal, for outputting a control signal from the output terminal inresponse to a result of comparison between voltages of the first andsecond input terminals, and a first current supply circuit having afirst terminal connected to a power source for supplying a supplyvoltage, a second terminal connected to the input terminal of the bufferamplifier, and a control signal input terminal connected to the secondoutput terminal of the first comparator, for controlling conduction andnon-conduction of a current flowing from the first terminal to thesecond terminal in response to the control signal.

A twelfth aspect of the invention relates to a pin electronics cardcomprising a connector connected to a test board on which an objectivedevice is mounted, an interface circuit having a signal input terminalfor interfacing to use a signal entered from the signal input terminalas an input to the tester, and a ringing preventive circuit connectedbetween the signal input terminal of the interface circuit and theconnector, for removing ringing caused in a signal entered from thesignal input terminal, wherein the ringing preventive circuit comprisesa buffer amplifier having an input terminal connected to the connector,and an output terminal connected to the signal input terminal, a firstcomparator having a first input terminal connected to the outputterminal of the buffer amplifier, a second input terminal provided witha first reference voltage, and an output terminal, for outputting acontrol signal from the output terminal in response to a result ofcomparison between voltages of the first and second input terminals, anda first current supply circuit having a first terminal connected to apower source for supplying a supply voltage, a second terminal connectedto the input terminal of the buffer amplifier, and a control signalinput terminal connected to the output terminal of the first comparator,for controlling on/off by the control signal and passing a current fromthe first terminal to the second terminal depending on the voltage ofthe output terminal.

A thirteenth aspect of the invention relates to a pin electronics cardof the twelfth aspect, further comprising a high impedance circuitinserted between the first input terminal of the buffer amplifier andthe connector, for setting the impedance at the connector side as seenfrom the buffer amplifier hither than before insertion.

A fourteenth aspect of the invention relates to a semiconductor devicehaving a ringing preventive circuit adjacent to an output pin forremoving ringing of pulse signal outputted from an output stage, whereinsaid ringing preventive circuit comprises a buffer amplifier having aninput terminal connected to the output stage, and an output terminalconnected to the output pin, a first comparator having a first inputterminal connected to the output terminal of the buffer amplifier, asecond input terminal provided with a first reference voltage, and anoutput terminal, for outputting a control signal from the outputterminal in response to a result of comparison between voltages of thefirst and second input terminals, and a first current supply circuithaving a first terminal connected to a power source for supplying asupply voltage, a second terminal connected to the input terminal of thebuffer amplifier, and a control signal input terminal connected to theoutput terminal of the first comparator, for controlling conduction andnon-conduction of a current flowing from the first terminal to thesecond terminal by the control signal.

A fifteenth aspect of the invention relates to a semiconductor devicehaving a ringing preventive circuit for removing ringing of pulse signaloutputted from an output stage disposed near an output pin, wherein saidringing preventive circuit comprises a buffer amplifier having an inputterminal connected to the connector, and an output terminal connected tothe signal input terminal, a first comparator having a first inputterminal connected to the output terminal of the buffer amplifier, asecond input terminal provided with a first reference voltage, and anoutput terminal, for outputting a control signal from the outputterminal in response to a result of comparison between voltages of thefirst and second input terminals, and a first current supply circuithaving a first terminal connected to the power source, a second terminalconnected to the input terminal of the buffer amplifier, a thirdterminal connected to the output terminal of the buffer amplifier, and acontrol signal input terminal connected to the output terminal of thefirst comparator, for controlling on/off by the control signal andpassing a current from the first terminal to the second terminaldepending on the voltage of the output terminal.

According to the first, eighth and eleventh aspects of the invention,the first comparator detects generation of ringing depending on whetherthe voltage of the output terminal of the buffer amplifier exceeds thefirst reference voltage or not. When ringing occurs, corresponding tothe output of the first comparator, the first current supply circuitsupplies a current to the input of the buffer amplifier, and the outputof the buffer amplifier is restrained from exceeding the first referencevoltage to increase the difference, thereby preventing ringing.

According to the second aspect of the invention, the constant currentpower source supplies a predetermined current to the switch means, andhence it is not necessary to adjust the element composing the firstcurrent supply circuit depending on the amplitude of the signal ormagnitude of ringing.

It is not necessary to adjust the element composing the first currentsupply circuit, and hence handling of the ringing preventive circuit iseasy.

According to the third aspect of the invention, the switch means has thefirst current electrode connected to the first power source, and thesecond current electrode to the first current output terminal of thecurrent mirror circuit, and therefore the first and second currentelectrodes are free from effects of input and output of the bufferamplifier, and hence it is not necessary to adjust the element composingthe first current supply circuit, and it is not influenced by, forexample, the base line of the pulse entered in the input terminal of thebuffer amplifier, and malfunction does not occur if the level of thesignal of input and output of the buffer amplifier is shifted so thathandling of the ringing preventive circuit may be easily performed.

According to the fourth aspect of the invention, the first and secondcomparators detect generation of ringing depending on whether thevoltage of the output terminal of the buffer amplifier exceeds the firstor second reference voltage or not. When ringing occurs, correspondingto the output of the first or second comparator, the first or secondcurrent supply circuit supplies a current to the input of the bufferamplifier, and the output of the buffer amplifier is restrained fromexceeding the first and second reference voltage to increase thedifference, so that ringing occurring in both rise and fall of a signalcan be suppressed.

According to the fifth, ninth and twelfth aspects of the invention, thefirst comparator detects generation of ringing depending on whether thevoltage of the output terminal of the buffer amplifier exceeds the firstreference voltage or not. When ringing occurs, the first current supplycircuit supplies a current corresponding to the output of the bufferamplifier to the input of the buffer amplifier, and the output of thebuffer amplifier is restrained from exceeding the first referencevoltage to increase the difference, so that ringing may be suppressed.

According to the sixth aspect of the invention, the first and secondcomparators detect generation of ringing depending on whether thevoltage of the output terminal of the buffer amplifier exceeds the firstor second reference voltage or not. When ringing occurs, the first andsecond current supply circuits supply a current corresponding to theoutput of the buffer amplifier to the input of the buffer amplifier, andthe output of the buffer amplifier is restrained from exceeding thefirst and second reference voltages to increase the difference, so thatringing may be suppressed.

According to the seventh aspect of the invention, the first comparatordetects generation of ringing depending on whether the voltage of theoutput terminal of the buffer amplifier exceeds the first referencevoltage or not. When ringing occurs, the first current supply circuitsupplies a current corresponding to the output of the buffer amplifieramplified by the differential amplifying circuit, to the input of thebuffer amplifier, and the output of the buffer amplifier is restrainedfrom exceeding the first reference voltage to increase the difference,so that ringing may be suppressed.

According to the tenth and thirteenth aspects of the invention, thefirst comparator detects generation of ringing depending on whether thevoltage of the output terminal of the buffer amplifier exceeds the firstreference voltage or not. When ringing occurs, the first current supplycircuit supplies a current between the high impedance converting circuitand input terminal of the buffer amplifier, corresponding to the outputof the first comparator, and the output of the buffer amplifier isrestrained from exceeding the first reference voltage to increase thedifference, and the current flowing when the first current supplycircuit supplies the current can be decreased by the high impedanceconverting circuit.

According to the fourteenth aspect of the invention, the firstcomparator detects generation of ringing depending on whether thevoltage of the output terminal of the buffer amplifier exceeds the firstreference voltage or not. When ringing occurs in the pulse signaltransmitted through a transmission path, the first current supplycircuit supplies a current to the input terminal of the bufferamplifier, corresponding to the output of the first comparator, and theoutput of the buffer amplifier is restrained from exceeding the firstreference voltage to increase the difference, and at this time, sincethe buffer amplifier is contained in the ringing preventive circuit, thebuffer amplifier used in the output of the semiconductor may be omittedby adding the ringing preventive circuit. Accordingly, ringing can besuppressed by the ringing preventive circuit, while limiting expansionof the circuit scale by adding the ringing preventive circuit.

According to the fifteenth aspect of the invention, the first comparatordetects generation of ringing depending on whether the voltage of theoutput terminal of the buffer amplifier exceeds the first referencevoltage or not. When ringing occurs in the pulse signal transmittedthrough a transmission path, the first current supply circuit supplies acurrent to the input terminal of the buffer amplifier depending on theoutput of the buffer amplifier, corresponding to the output of the firstcomparator, and the output of the buffer amplifier is restrained fromexceeding the first reference voltage to increase the difference, and atthis time, since the buffer amplifier is contained in the ringingpreventive circuit, the buffer amplifier used in the output of thesemiconductor may be omitted by adding the ringing preventive circuit.Accordingly, the ringing can be suppressed by the ringing preventivecircuit, while limiting expansion of the circuit scale by adding theringing preventive circuit.

It is therefore an object of the present invention to provide a ringingpreventive circuit capable of preventing malfunction in circuit andothers in the subsequent stages by removing ringing at a specifiedposition, if ringing or other noise should occur in the process oftransmission of signal through a transmission path, in order to solvethe problems in the prior art, and also to provide a pin electronicscard of a semiconductor testing apparatus incorporating such ringingpreventive circuit, DUT board incorporating a ringing preventivecircuit, and a semiconductor device incorporating a ringing preventivecircuit.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following description ofthe present invention when taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the constitution of a DUT boardaccording to a first preferred embodiment of the invention connected toa pin electronics card;

FIG. 2 is a waveform diagram showing an output signal from a ringingpreventive circuit;

FIG. 3 is block diagram showing the constitution of a pin electronicscard according to a second preferred embodiment of the inventionconnected to a DUT board;

FIG. 4 is block diagram showing the constitution of a DUT boardaccording to a third preferred embodiment of the invention connected toa pin electronics card;

FIG. 5 is a circuit diagram showing an example of constitution of a highimpedance converting circuit of FIG. 4;

FIG. 6 is block diagram showing the constitution of a pin electronicscard according to a fourth preferred embodiment of the inventionconnected to a DUT board;

FIG. 7 is block diagram showing the constitution of a semiconductordevice according to a fifth preferred embodiment of the invention;

FIG. 8 is block diagram showing the constitution of a ringing preventivecircuit;

FIG. 9 is block diagram showing another constitution of the ringingpreventive circuit;

FIG. 10 is a circuit diagram showing a first example of constitution ofa buffer amplifier used in the invention;

FIG. 11 is a circuit diagram showing a second example of constitution ofa buffer amplifier used in the invention;

FIG. 12 is a circuit diagram showing a third example of constitution ofa buffer amplifier used in the invention;

FIG. 13 is a circuit diagram showing the constitution of a ringingpreventive circuit according to a first preferred embodiment of thecircuit of FIG. 8;

FIG. 14 is a circuit diagram showing the constitution of a ringingpreventive circuit according to a second preferred embodiment of thecircuit FIG. 8;

FIG. 15 is a circuit diagram showing the constitution of a ringingpreventive circuit according to third preferred embodiment of thecircuit FIG. 8;

FIG. 16 is a circuit diagram showing the constitution of a ringingpreventive circuit according to a fourth preferred embodiment of thecircuit FIG. 8;

FIG. 17 is a circuit diagram showing the constitution of a ringingpreventive circuit according to a fifth preferred embodiment of thecircuit FIG. 8;

FIG. 18 is a circuit diagram showing the constitution of a ringingpreventive circuit according to a sixth preferred embodiment of thecircuit FIG. 8;

FIG. 19 is circuit diagram showing the constitution of a ringingpreventive circuit according to a seventh preferred embodiment of thecircuit FIG. 8;

FIG. 20 is a circuit diagram showing the constitution of a ringingpreventive circuit according to an eighth preferred embodiment of thecircuit FIG. 8;

FIG. 21 is a circuit diagram showing the constitution of a ringingpreventive circuit according to a ninth preferred embodiment of thecircuit FIG. 8;

FIG. 22 is a circuit diagram showing the constitution of a ringingpreventive circuit according to a tenth preferred embodiment of thecircuit FIG. 8;

FIG. 23 is a circuit diagram showing the constitution of a ringingpreventive circuit according to an eleventh preferred embodiment of thecircuit FIG. 8;

FIG. 24 is a circuit diagram showing the constitution of a ringingpreventive circuit according to a twelfth preferred embodiment of thecircuit FIG. 8;

FIG. 25 is a block diagram showing the connection state of aconventional DUT board and a conventional testing apparatus for testingan objective device when testing a semiconductor device;

FIG. 26 is a block diagram showing the constitution of a conventionalpin electronics card;

FIG. 27 is a waveform diagram of an ideal signal free from ringing orother noise and an output of pin electronics card when processing suchsignal;

FIG. 28 is a waveform diagram of a signal containing ringing or othernoise and an output of pin electronics card when processing such signal;

FIG. 29 is a block diagram showing the constitution of a conventionalsemiconductor device;

FIG. 30 is a waveform diagram for explaining the operation of a ringingpreventive circuit;

FIG. 31 is a waveform diagram showing the output of a ringing preventivecircuit of the invention;

FIG. 32 is a circuit diagram showing a fourth example of constitution ofa buffer amplifier used in the invention; and

FIG. 33 is a circuit diagram showing a fifth example of constitution ofa buffer amplifier used in the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first preferred embodiment of the invention is described below byreferring to FIG. 1 and FIG. 2. FIG. 1 is a conceptual diagram forexplaining the constitution of a DUT board in the first preferredembodiment of the invention. FIG. 1 shows the connection relation fortesting the semiconductor device, relating to a DUT board 20 accordingto the first preferred embodiment incorporating a ringing preventivecircuit 21, and a pin electronics card 7 of a conventional semiconductortesting apparatus. In FIG. 1, reference numeral 21 is a ringingpreventive circuit being adjacent to a connector 5 of the DUT board 20,and comprising an input terminal 21a connected to a transmission path 4,and an output terminal 21b connected to the connector 5, and otherreference numerals same as in FIG. 25 represent same or correspondingparts as in FIG. 25.

The ringing preventive circuit 21 is provided in the location adjacentto the connector 5 in order to prevent ringing before the signaloutputted from the output terminal 21b of the ringing preventive circuit21 reaches the connector 5.

The operation is explained below. It assumes that the signal from theoutput circuit of an objective device 1 is an output signal only, and arelay K1 in the pin electronics card 7 of the semiconductor testingapparatus is supposed to be in ON (closed) state. For example, it issupposed as follows: a waveform (L level voltage V_(L), H level voltageV_(H)) shown in (a) of FIG. 27 is issued from the output circuit of theobjective device 1, and is mixed with noise such as ringing in theprocess of passing through the transmission path 4, and it becomes awaveform as shown in (a) of FIG. 28 at the input terminal 21a of theringing preventive circuit 21. Herein, the voltage at the input terminal21a is supposed to be V_(ri), and the voltage V_(ri) is a voltage on thebasis of GND. The lower side logic level is L level, and the higher sidelogic level is H level.

The ringing preventive circuit 21 operates when V_(ri) >V_(H) +ΔV andV_(ri) <V_(L) -ΔV, and supplies a current for suppressing overshoot ofshaded areas A, C shown in (a) of FIG. 28. The detail of the ringingpreventive circuit 21 is shown in preferred embodiment 6 and after. Whenthe ringing preventive circuit 21 operates in order to suppressovershoot of shaded areas A, C, the damping factor increased, andoscillations of shaded areas B, D shown in (a) of FIG. 28 become small,and the peak of voltage fluctuations corresponding to B, C approachesvoltages V_(H), V_(L), respectively.

After passing through such ringing preventive circuit 21, the waveformappearing at the output terminal 21b of the ringing preventive circuit21 becomes a waveform suppressed of ringing as shown in FIG. 2.Afterwards, it is put into an input terminal 11a of a pin electronicscomparator/driver 10. The voltage V_(ri) of the input signal is comparedwith threshold voltages V_(Hth), V_(Lth) of the pin electronicscomparator 11 as shown in FIG. 26. As a result of comparison, the pinelectronics comparator 11 outputs the waveform shown in (b) of FIG. 27from an output terminal 11b of the pin electronics comparator 11, and apin electronics control circuit 13 receives this output, and convertsinto a signal that can be processed in a semiconductor testing apparatus6 shown in FIG. 25, and transmits inside.

By inserting such ringing preventive circuit 21 in a DUT board 20 offront stage of the pin electronics card 7 of the semiconductor testingapparatus, noise such as ringing inserted in the transmission path 4 canbe removed, so that malfunction of the pin electronics comparator may beprevented.

A second preferred embodiment of the invention is described by referenceto FIG. 3. FIG. 3 is a conceptual diagram for explaining theconstitution of a pin electronics card according to the second preferredembodiment of the invention. FIG. 3 shows the connection relation fortesting of semiconductor device, relating to a conventional DUT board 2and a pin electronics card 22 of the semiconductor testing apparatus ofthe second preferred embodiment.

In FIG. 3, reference numeral 21 denotes a ringing preventive circuithaving an input terminal 21a connected to the connector 8 through therelay K1 adjacent to the input terminal (the connector 8) of the pinelectronics card 22, and an output terminal 21b connected to the inputterminal 11a of the pin electronics comparator/driver 10, K2 is a relayhaving one end connected to the connector 8, and other end connected tothe output terminal 12b of the pin electronics driver 12, and otherreference numerals same as in FIG. 26 represent same or correspondingparts in FIG. 26.

What the pin electronics card 22 shown in FIG. 3 differs from the pinelectronics card shown in FIG. 26 lies in that the ringing preventivecircuit 21 is inserted between the relay K1 and input terminal 11a ofthe pin electronics comparator 11, and that the output terminal 12b ofthe pin electronics driver 12 is connected to the connector 8 throughthe relay K2. That the output terminal 12b of the pin electronics driver12 is connected to the connector through the relay K2 is intended not toconnect the input terminal 21a of the ringing preventive circuit 21 andthe output terminal 12b of the pin electronics driver 12 when the pinelectronics driver 12 operates. If the voltage at the output terminal12b of the pin electronics driver 12 reaches the operating voltage ofthe ringing preventive circuit 21, it is intended so that the ringingpreventive circuit 21 may not operate to supply current. It is thereforenecessary to operate the relays K1, K2 complementarily. The ringingpreventive circuit 21 should be preferably adjacent to the inputterminal 11a of the pin electronics comparator 11 so that ringing maynot occur in its output.

The operation is described below. Suppose that the signal from theoutput circuit 8 of the semiconductor device is output only, and thatthe relay K2 and relay K1 in the pin electronics card 22 of thesemiconductor testing apparatus are respectively in OFF (open) and ON(closed) state.

For example, it is supposed as follows: a waveform (L level voltageV_(L), H level voltage V_(H)) shown in (a) of FIG. 27 is issued from theoutput circuit of the objective device 1, and is mixed with noise suchas ringing in the process of passing through the transmission path 4,and it becomes a waveform as shown in (a) of FIG. 28 when transmitted tothe input terminal 21a of the ringing preventive circuit 21. Herein, thevoltage at the input terminal 21a is supposed to be V_(ri), and thevoltage V_(ri) is a voltage on the basis of GND. The ringing preventivecircuit 21 operates when V_(ri) >V_(H) +ΔV and V_(ri) <V_(L) -ΔV, andsupplies a current for suppressing overshoot of shaded areas A, C shownin (a) of FIG. 28. The detail of the ringing preventive circuit 21 isshown in embodiments of FIG. 8 and after. When the ringing preventivecircuit 21 operates in order to suppress overshoot of shaded areas A, C,the damping factor increases, and oscillations of shaded areas B, Dshown in (a) of FIG. 28 become small, and the peak of voltagefluctuations corresponding to B, D approaches voltages V_(H), V_(L),respectively.

After passing through such ringing preventive circuit 21, the waveformappearing at the output terminal 21b of the ringing preventive circuit21 becomes a waveform suppressed of ringing as shown in FIG. 2.Afterwards, it is put into an input terminal 11a of a pin electronicscomparator 11. The voltage V_(ri) of the input signal is compared withthreshold voltages V_(Hth), V_(Lth) of the pin electronics comparator 11as shown in FIG. 25. As a result of comparison, the pin electronicscomparator 11 outputs the waveform shown in (b) of FIG. 27 from anoutput terminal 11b of the pin electronics comparator 11, and a pinelectronics control circuit 13 receives this output, and converts into asignal that can be processed in a semiconductor testing apparatus 6shown in FIG. 25, and transmits inside.

By inserting such ringing preventive circuit 21 in a front stage of theinput terminal 11a of the pin electronics comparator 11 of thesemiconductor testing apparatus, noise such as ringing inserted in thetransmission path 4 can be removed, so that malfunction of the pinelectronics comparator 11 may be prevented. In addition, byincorporating the ringing preventive circuit 21 in the pin electronicscard 22 of the semiconductor testing apparatus, it is not necessary toinstall the ringing preventive circuit 21 in all DUT boards prepared inevery application corresponding to the type of the objective device.

A third preferred embodiment of the invention is described below byreference to FIG. 4 and FIG. 5. FIG. 4 is a conceptual diagram forexplaining the constitution of a pin electronics card according to thethird preferred embodiment of he invention. FIG. 4 shows the connectionrelation of testing of the semiconductor device, relating to the ringingpreventive circuit 21 of the third preferred embodiment of theinvention, the DUT board 23 incorporating a high impedance convertingcircuit 24, and the pin electronics card 7 of the conventionalsemiconductor testing apparatus.

The DUT board 23 shown in FIG. 4 is the DUT board 20 of the firstpreferred embodiment plus the high impedance converting circuit 24. Thehigh impedance converting circuit 24 is inserted before the ringingpreventive circuit 21. The high impedance converting circuit 24comprises one end 24a connected to the transmission path 4 being builtin the DUT board 23, and other end 24b connected to the input terminal21a of the ringing preventive circuit 21. In FIG. 4, same referencenumerals as in FIG. 1 denote the same corresponding parts as in FIG. 1.By inserting the high impedance converting circuit 24 before the ringingpreventive circuit 21, the impedance when seeing the transmission path 4side from the ringing preventive circuit 21 is higher than beforeinsertion of the high impedance converting circuit 24.

The operation is explained below. The signal from the output circuit ofthe objective device 1 is an output signal only, and the relay K1 in thepin electronics card 7 of the semiconductor testing apparatus issupposed to be in ON (closed) state.

Suppose the waveform shown in (a) of FIG. 27 is issued from the outputcircuit of the objective device 1, pass through the transmission path 4,is converted into high impedance in the high impedance convertingcircuit 24, and mixed with noise such as ringing in the process ofpassing through the transmission path 4, and it becomes a waveform asshown in (a) of FIG. 28 at the input terminal 21a of the ringingpreventive circuit.

The ringing preventive circuit 21 operates same as in the firstpreferred embodiment, that is, the ringing preventive circuit 21operates when V_(ri) >V_(H) +ΔV and V_(ri) <V_(L) -ΔV, upon input of thesignal shown in (a) of FIG. 28 at the input terminal 21a of the ringingpreventive circuit 21.

Afterwards, it is put into the input terminal 11a of the pin electronicsdriver 12 comparator 10 in FIG. 3, and is compared with thresholdvoltages V_(Hth), V_(Lth) of the pin electronics comparator 11 as shownin FIG. 26. The compared waveforms shown in (b) of FIG. 27 are sent outinto the pin electronics control circuit 13 through the output terminal11b of the pin electronics comparator 11.

By inserting such ringing preventive circuit 21 in the DUT board 23connected to the front stage of the pin electronics card 7 of thesemiconductor testing apparatus, noise such as ringing inserted in thetransmission path 4 can be removed, so that malfunction of the pinelectronics comparator 11 in the pin electronics comparator/driver 10may be prevented.

By inserting the high impedance converting circuit 24 before the ringingpreventive circuit 21, when the ringing preventive circuit 21 operatesto supply current, the current flowing into the transmission path 4becomes smaller. Accordingly, the current supplyback amount of theringing preventive circuit 21 can be decreased. An example of highimpedance converting circuit is shown in FIG. 5. The high impedanceconverting circuit shown in FIG. 5 has a resistance R1 connected betweenone end 24a and other end 24b.

A fourth preferred embodiment of the invention is described below byreference to FIG. 2, FIG. 5 and FIG. 6. FIG. 6 is a conceptual diagramfor explaining the constitution of a pin electronics card according tothe fourth preferred embodiment of the invention. FIG. 6 shows theconnection relation of testing of the semiconductor device, relating tothe conventional DUT board 2 and the pin electronics card 25 of thesemiconductor testing apparatus according to the fourth preferredembodiment.

The pin electronics card 25 shown in FIG. 6 is composed of the pinelectronics card 22 shown in FIG. 3 and the high impedance convertingcircuit 24 inserted before the ringing preventive circuit 21 shown inFIG. 3.

The high impedance converting circuit 24 comprises one end 24a connectedto the other end of the relay K1 incorporated in the pin electronicscard 25, and other end 24b connected to the input terminal 21a of theringing preventive circuit 21. In FIG. 6, same reference numerals as inFIG. 3 denote the same corresponding parts as in FIG. 3. By insertingthe high impedance converting circuit 24 before the ringing preventivecircuit 21, the impedance of the connector 8 of the pin electronics card25 viewed from the ringing preventive circuit 21 is higher than beforeinsertion of the high impedance converting circuit 24.

The operation is explained below. The signal from the output circuit ofthe objective device 1 is an output only, and the relay K2 and relay K1in the pin electronics card 25 of the semiconductor testing apparatusare supposed to be in OFF (open), and 0N (closed) state, respectively.

The ringing preventive circuit 21 operates same as in the secondpreferred embodiment, that is, the ringing preventive circuit 21operates when V_(ri) >V_(H) +ΔV and V_(ri) <V_(L) -ΔV, upon input of thesignal shown in (a) of FIG. 28 at the input terminal 21a of the ringingpreventive circuit 21.

After passing through such ringing preventive circuit 21, the waveformappearing at the output terminal 21b of the ringing preventive circuit21 becomes a waveform suppressed in ringing as shown in FIG. 2.Afterwards, it is put into the input terminal 11a of the pin electronicscomparator 11, and the voltage V_(ri) of the input signal is comparedwith threshold voltages V_(Hth), V_(Lth) of the pin electronicscomparator 11 as shown in FIG. 25. As a result of comparison, the pinelectronics comparator 11 outputs the waveform shown in (b) of FIG. 27from the output terminal 11b of the pin electronics comparator 11, andthe pin electronics control circuit 13 receives its output, and convertsit into a signal that can be processed in the semiconductor testingapparatus shown in FIG. 25, and transmits inside.

By inserting such ringing preventive circuit 21 before the inputterminal of the pin electronics comparator 11 of the pin electronicscomparator/driver 10, noise such as ringing mixed in the transmissionpath 4 can be removed, so that malfunction of the pin electronicscomparator 11 may prevented.

By incorporating the ringing preventive circuit 21 into the pinelectronics card 25 of the semiconductor testing apparatus, it is notnecessary to install the ringing preventive circuit 21 in all DUT boardsprepared in every application corresponding to the type of the objectivedevice, and it is economical.

By inserting the high impedance converting circuit 24 before the ringingpreventive circuit 21, when the ringing preventive circuit 21 operatesto supply current, the current flowing out from the connector 8 becomessmaller. Accordingly, the current supplyback amount of the ringingpreventive circuit 21 can be decreased. What is shown in FIG. 5 is usedin the high impedance converting circuit 24.

A semiconductor device according to a fifth preferred embodiment of theinvention is described below by reference to FIG. 7. FIG. 7 is aconceptual diagram showing the constitution of the semiconductor deviceof the fifth preferred embodiment of the invention. In the semiconductordevice 26 shown in FIG. 7, the ringing preventive circuit 21 is adjacentto the output pin 15. What the semiconductor device 26 of the fifthpreferred embodiment differs from the semiconductor device 14 shown inFIG. 29 is that the ringing preventive circuit 21 is used instead of theoutput buffer 16. Inside the ringing preventive circuit 21, as explainedin the embodiment of FIG. 8 and after, a buffer amplifier is providedbetween input terminal 21a and output terminal 21b, so that the outputbuffer can be omitted.

In FIG. 7, reference numeral 26 is the semiconductor device, 15 is theoutput pin of the semiconductor device, 21 is the ringing preventivecircuit having the input terminal 21a and output terminal 21b connectedto the output pin 15, for giving an output signal being rid of ringingto the output pin 15, 17 is the transmission path connected to the inputterminal 21a of the ringing preventive circuit 21, and 18 is a circuitbefore an output stage for generating a signal to be transmitted to thetransmission path 17. The transmission path 17 is a wiring formed in thesemiconductor device 26, which may include wiring of aluminum,polysilicon, etc., and the output pin 15 may be a pad or the like.

The operation is described below. From the circuit 18 before the outputstage of the semiconductor device 26, for example, an output signalhaving a waveform shown in (a) of FIG. 27 is outputted, and it istransmitted to the input terminal 21a of the ringing preventive circuit21 through the transmission path 17 composed of internal wiring of thesemiconductor device 26. The output signal is mixed with noise such asringing while passing through the transmission route 17 and others, andit is supposed to become the waveform shown in (a) of FIG. 28 whenreaching the input terminal 21a of the ringing preventive circuit.Supposing the voltage at the input terminal 21a to be V_(ri), thevoltage V_(ri) is supposed to be a voltage on the basis of GND.

The ringing preventive circuit 21 operates when V_(ri) >V_(H) +ΔV andV_(ri) <V_(L) -ΔV, and supplies a current for suppressing overshoot ofshaded areas A, C shown in (a) of FIG. 28. Details of preferredembodiments of the ringing preventive circuit 21 are shown inembodiments 1 and after. When the ringing preventive circuit 21 operatesin order to suppress overshoot of shaded areas A, C, the damping factorincreases, and oscillations of shaded areas B, D shown in (a) of FIG. 28decrease, and the vibration peak voltage approaches voltages V_(H),V_(L), respectively. Afterwards, from the output terminal 21b of theringing preventive circuit 21, the output signal having the waveform asshown in FIG. 2 is transmitted to the output pin 15 of the semiconductordevice 26.

By inserting such ringing preventive circuit 21 to the output unit ofthe semiconductor device 26, it is possible to remove the noise, such asringing, mixed in the transmission path 17.

The ringing preventive circuit according to the invention is describedbelow by referring to FIG. 8 to FIG. 12, FIG. 32, and FIG. 33. FIG. 8 isa block diagram showing the constitution of the ringing preventivecircuit 21. In FIG. 8, reference numeral 30 is a buffer amplifier havingan input terminal connected to the input terminal 21a of the ringingpreventive circuit 21, and an output terminal connected to the outputterminal 21b of the ringing preventive circuit 21, 31 is a lowerpotential side comparator (hereinafter called L side comparator) havingan inverting input terminal connected to the output terminal of thebuffer amplifier 30, a non-inverting input terminal connected to areference power source E1, and an output terminal, for comparing if theoutput of the buffer amplifier 30 is smaller than the voltage of thereference power source E1 or not, 33 is a lower potential side currentsupply circuit (hereinafter called L side current supply circuit) forsupplying current to an input terminal of the buffer amplifier 30 inresponse to the signal given from the output terminal of the L sidecomparator 31, 32 is a higher potential side comparator (hereinaftercalled H side comparator) having an inverting input terminal connectedto the output terminal of the buffer amplifier 30, a non-inverting inputterminal connected to a reference power source E2, and an outputterminal, for comparing if the output of the buffer amplifier 30 islarger than the voltage of the reference power source E2 or not, 34 is ahigher potential side current supply circuit (hereinafter called H sidecurrent supply circuit) for supplying current flowing out from the inputterminal of the buffer amplifier 30 in response to the signal given fromthe output terminal of the H side comparator 32, and others of the samereference numerals as in FIG. 1 are same or corresponding parts as inFIG. 1. The buffer amplifier 30 is usually used as gain 1. However, thegain of the buffer amplifier 30 is determined depending on thespecification of the circuit connected to the input and the circuitconnected to the output, and hence it is not limited to 1.

The operation is described below. The voltage at the input terminal 21aof the ringing preventive circuit 21 is supposed to be voltage V_(ri) onthe basis of GND, the voltage (base line) converged at the L side of thewaveform mixed with noise such as ringing to be voltage V_(L) on thebasis of GND, the voltage (top line) converged at the H side of thewaveform mixed with noise such as ringing to be voltage V_(H) on thebasis of GND, the voltage of the reference power source E1 to be V_(L)-ΔV, the voltage of the reference power source E2 to be V_(H) +ΔV. Amongthese voltages, there is a relation of V_(L) -ΔV<V_(L) <V_(H) <V_(H)+ΔV, where ΔV is set as ΔV>Vos, supposing the maximum offset voltage ofthe L side comparator 31 and H side comparator 32 to be Vos.

When the waveform mixed with ringing or other noise shown in (a) of FIG.28 is put in the input terminal 21a of the ringing preventive circuit21, at the output side of the ringing preventive circuit 21, theimpedance is low in the buffer amplifier 30, and the output voltage ofthe buffer amplifier 30 having the gain equal to 1 is V_(ri).

When V_(ri) <V_(L) -ΔV, the output of the L side comparator 31 is Hlevel, and the L side current supply circuit 33 is operated, and acurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) >V_(L) -ΔV, the output of the L side comparator 31 is Llevel, and the L side current supply circuit 33 is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) <V_(H) +ΔV, the output of the H side comparator 32 is Hlevel, and the H side current supply circuit 34 is not operated, an nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) >V_(H) +ΔV, the output of the H side comparator 32 is Llevel, and the H side current supply circuit 34 is operated, and acurrent (negative current) is supplied to the front stage of the bufferamplifier 30.

The above operations are summed up in Table 1.

                  TABLE 1                                                         ______________________________________                                                                         L side                                                                              H side                                               Output of                                                                              Output of current                                                                             current                                Ringing preventive circuit                                                                  L side   H side    supply                                                                              supply                                 input voltage V.sub.ri                                                                      comparator                                                                             comparator                                                                              circuit                                                                             circuit                                ______________________________________                                        V.sub.ri > V.sub.H + ΔV                                                               L        L         OFF   ON                                     V.sub.H + ΔV > V.sub.ri > V.sub.H - ΔV                                          L        H         OFF   OFF                                    V.sub.L - ΔV > V.sub.ri                                                               H        H         ON    OFF                                    ______________________________________                                    

In this way, the ringing preventive circuit 21 eliminates the shadedareas A, C in (a) of FIG. 28. At this time, the shaded areas B, D aresmaller in oscillation as the damping factor increases when removing theshaded areas A, C, and hence the peak voltage of oscillation approachesV_(H) and V_(L). The waveform of the output terminal 21b after passingthrough such ringing preventive circuit 21 is a waveform having a rangefrom V_(H) +ΔV to V_(L) -ΔV as shown in FIG. 2.

After the transmission path for transmitting the digital waveform, whenthis ringing preventive circuit 21 is inserted, voltage noise of ringingor the like is prevented, and malfunction of the subsequent circuits canbe suppressed as far as possible.

The buffer amplifier is described below while referring to FIG. 10 toFIG. 12, FIG. 32, and FIG. 33. FIG. 10 is a circuit diagram showing theconstitution of a buffer amplifier including emitter follower. FIG. 10shows different circuit compositions of the constitution of the bufferamplifier.

In (a) of FIG. 10, reference numeral 35 is an input terminal of thebuffer amplifier, 45 is an output terminal of the buffer amplifier, Q80is an npn type bipolar transistor having a base connected to the inputterminal 35, an emitter provided with a negative supply voltage Em, anda collector, 36 is a constant current source connected to a nodeprovided with a positive supply voltage Ep for producing a specificcurrent I1, R50 is a resistance having one end connected to the nodeprovided with the positive supply voltage Ep through the constantcurrent source 36 and other end connected to the collector of the npntype bipolar transistor Q80, Q81 is an npn type bipolar transistorhaving a base connected to the collector of the npn type bipolartransistor Q80, an emitter provided with the negative supply voltage Em,and a collector connected to the output terminal 45 of the bufferamplifier, 37 is a constant current source connected to the nodeprovided with the positive supply voltage Ep for producing a specificcurrent I2, and R51 is a resistance having one end connected to theconstant current source 37 and other end connected to the outputterminal 45.

In (b) of FIG. 10, reference numeral 35 is an input terminal of thebuffer amplifier, 45 is an output terminal of the buffer amplifier, Q82is an npn type bipolar transistor having a base connected to the inputterminal 35, an emitter provided with a negative supply voltage Em, anda collector, R52 is a resistance having one end provided with a positivesupply voltage Ep and other end connected to the collector of thetransistor Q82, Q83 is an npn type bipolar transistor having a baseconnected to the collector of the transistor Q82, an emitter providedwith the negative supply voltage Em, and a collector connected to theoutput terminal 45, and R53 is a resistance having one end connected tothe node provided with the positive supply voltage Ep and other endconnected to the output terminal 45.

The buffer amplifiers shown in FIG. 10 are mainly applicable to ringingat the lower potential side. To ringing at the higher potential side,the output current of the ringing preventive circuit 21 is applicable ina range of I2 or less.

In addition, the same circuit as shown in FIG. 10 can be composed byusing a MOS transistor. FIG. 32 is a circuit diagram showing theconstitution of a buffer amplifier of source follower type. FIG. 32shows different circuit compositions of buffer amplifier.

In (a) of FIG. 32, reference numeral 35 is an input terminal of thebuffer amplifier, 45 is an output terminal of the buffer amplifier, Q90is an n channel MOS transistor having a gate connected to the inputterminal 35, a source provided with a negative supply voltage Em, and adrain, 76 is a constant current source connected to a node provided witha positive supply voltage Ep for producing a specific current I1, R60 isa resistance having one end connected to the node provided with thepositive supply voltage Ep through the constant current source 76 andother end connected to the drain of the n channel MOS transistor Q90,Q91 is an n channel MOS transistor having a gate connected to the drainof the n channel MOS transistor Q90, a source provided with the negativesupply voltage Em, and a drain connected to the output terminal 45 ofthe buffer amplifier, 77 is a constant current source connected to thenode provided with the positive supply voltage Ep for producing aspecific current I2, and R61 is a resistance having one end connected tothe constant current source 77 and other end connected to the outputterminal 45.

In (b) of FIG. 32, reference numeral 35 is an input terminal of thebuffer amplifier, 45 is an output terminal of the buffer amplifier, Q92is an n channel MOS transistor having a gate connected to the inputterminal 35, a source provided with a negative supply voltage Em, and adrain, R62 is a resistance having one end provided with a positivesupply voltage Ep and other end connected to the drain of the transistorQ92, Q93 is an n channel MOS transistor having a gate connected to thedrain of the transistor Q92, a source provided with the negative supplyvoltage Em, and a drain connected to the output terminal 45, and R63 isa resistance having one end connected to the node provided with thepositive supply voltage Ep and other end connected to the outputterminal 45. The buffer amplifiers shown in FIG. 32 are mainlyapplicable to ringing at the lower potential side. To ringing at thehigher potential side, the output current of the ringing preventivecircuit 21 is applicable in a range of I2 or less.

FIG. 11 and FIG. 12 are conceptual diagram and circuit diagram showingthe constitution of the buffer amplifier using an operational amplifier.In (a) of FIG. 11, reference numeral 35 is an input terminal of thebuffer amplifier, 45 is an output terminal of the buffer amplifier, 38is a differential input stage having a non-inverting input terminalconnected to the input terminal 35, an inverting input terminalconnected to the output terminal 45, and an output terminal 40, and 41is an output stage having an input terminal 42 connected to the outputterminal 40 of the differential input stage and the output terminal 45.

The circuit composition of the output stage 41 is shown in (b) of FIG.11. The output stage shown in (b) of FIG. 11 is a push-pull amplifier.In (b) of FIG. 11, reference numeral Q84 is an npn type bipolartransistor having a base connected to the input terminal 42, an emitterprovided with a negative supply voltage Em, and a collector, D10 is adiode having a cathode connected to the collector of the transistor Q84,and an anode, 43 is a constant current power source connected to thenode provided with a positive supply voltage Ep for producing a specificcurrent I, D11 is a diode having a cathode connected to the anode of thediode D10, and an anode connected to the constant current source 43, Q85is a pnp type bipolar transistor having a base connected to thecollector of the transistor Q84, a collector provided with the negativesupply voltage Em, and an emitter connected to the output terminal 45,and Q86 is an npn type bipolar transistor having a collector providedwith the positive supply voltage Ep, an emitter connected to the outputterminal 45, and a base connected to the anode of the diode D11.

The circuit composition of the output stage 41 shown in (a) of FIG. 11may be as shown in FIG. 12. The output stage shown in (a) of FIG. 12 isan example of totem pole type output stage. In (a) of FIG. 12, referencenumeral Q87 is an npn type bipolar transistor having a base connected tothe input terminal 42, an emitter provided with a negative supplyvoltage Em, and a collector, D12 is a diode having a cathode connectedto the collector of the transistor Q87, and an anode connected to theoutput terminal 45, R55 is a resistance having one end provided with apositive supply voltage Ep and other end, and Q88 is an npn type bipolartransistor having a base connected to the other end of the resistanceR55, a collector provided with the positive supply voltage Ep, and anemitter connected to the output terminal. Herein, the other end of theresistance R55 and the cathode of the diode D12 are connected.

The output stage shown is (b) of FIG. 12 is other example of totem poleamplifier. In (b) FIG. 12, same reference numerals as in (a) of FIG. 12are same or corresponding parts in (a) of FIG. 12. In (b) of FIG. 12,reference numeral Q89 is an npn type bipolar transistor having a baseconnected to the input terminal 42, an emitter provided with a negativesupply voltage Em, and a collector connected to the base of thetransistor Q87, R56 is a resistance connected between the outputterminal 45 and input terminal 42, and D13 is a diode having an anodeconnected to the other end of the resistance R55 and a cathode connectedto the cathode of the diode D12. In the output stage in (a) of FIG. 12,the other end of the resistance R55 and the cathode of the diode D12 areshort-circuited, whereas in the output stage shown in (b) of FIG. 12,the other end of the resistance R55 and the cathode of the diode D12 areconnected together through the diode D13.

FIG. 33 is a circuit diagram showing the constitution of a bufferamplifier using an operational amplifier. The other examples of thecircuit composition of the output stage 41 shown in (a) of FIG. 11 areshown n channel MOS transistor. 33, reference numeral Q94 is an nchannel MOS transistor having a gate connected to the input terminal 42,a source provided with a negative supply voltage Era, and a drain, 91 isa constant current source connected to the node provided with a positivesupply voltage Ep for producing a specific current I to the drain of thetransistor Q94, Q95 is a p channel MOS transistor having a gateconnected to the drain of the transistor Q94, a drain provided with thenegative supply voltage Em, and a source connected to the outputterminal 45, and Q96 is an n channel MOS transistor having a drainprovided with the positive supply voltage Ep, source connected to theoutput terminal 45, and a gate connected to the drain of the transistorQ94.

In (b) of FIG. 33, reference numeral Q97 is an n channel MOS transistorhaving a gate connected to the input terminal 42, a source and a drain,Q98 is an n channel MOS transistor having a source provided with anegative supply voltage Em, a drain connected to the source of thetransistor Q97, and a gate connected to the source of the transistorQ97, 96 is a constant current source for supplying a specific currentfrom the connection point provided with a positive supply voltage Ep tothe drain of the transistor Q97, Q99 is an n channel MOS transistorhaving a drain provided with the positive supply voltage Ep, a gateconnected to the drain of the transistor Q97, and a source connected tothe output terminal, and Q100 is an n channel MOS transistor having asource provided with the negative supply voltage Em, a gate connected tothe source of the transistor Q97, and a drain connected to the outputterminal 45.

In a preferred embodiment, the ringing preventive circuit capable ofremoving ringing occurring on the basal line and the top line of thepulse simultaneously is explained, but, for example, if ringing on thetop line only is a problem, as shown in FIG. 9, the L side comparator 31and L side current supply circuit 33 can be omitted from the ringingpreventive circuit 21 shown in FIG. 8. The same is said of the basalline side.

A first embodiment of the ringing preventive circuit of FIG. 8 of theinvention is described below by referring to FIG. 13.

In FIG. 13, reference numeral 48 is a positive power source terminal ofa ringing preventive circuit 21A, 49 is a negative power source terminalof a ringing preventive circuit 21A, 50 is an operational amplifierhaving a non-inverting input terminal connected to an input terminal21a, an inverting input terminal connected to an output terminal 21b,and an output terminal connected to the output terminal 21b, R2 is aresistance having one end connected to the positive power sourceterminal 48, and other end, Q1 is an npn type bipolar transistor havinga collector connected to the other end of the resistance R2, an emitterconnected to the non-inverted input terminal of the operationalamplifier 50, and a base connected to the output terminal of an L sidecomparator 31, R3 is a resistance having one end connected to thenegative power source terminal 49, and other end, Q2 is a pnp typebipolar transistor having a collector connected to the other end of theresistance R3, an emitter connected to the non-inverted input terminalof the operational amplifier 50, and a base connected to the outputterminal of an H side comparator 32, and other reference numerals sameas in FIG. 8 denote same or corresponding parts as in FIG. 8. The L sidecomparator 31 and H side comparator 32 shown in FIG. 13 are bothconnected to the positive power source terminal 48 and negative powersource terminal 49, and are operated by the voltage supplied from thepositive power source terminal 48 and negative power source terminal 49to the ringing preventive circuit 21A.

As shown in FIG. 13, the L side current supply circuit 33A is composedof resistance R2 and npn transistor Q1. The H side current supplycircuit 34A is composed of resistance R3 and pnp type transistor Q2. InFIG. 13, the buffer amplifier 30 is composed of the voltage followerusing an operational amplifier as shown in (a) of FIG. 11, but it may becomposed otherwise, and, for example, it may be also composed by usingan emitter following as shown in FIG. 10, and the same effects arebrought about.

The operation is described below. The voltages V_(ri), V_(L), V_(H),V_(L) -ΔV, V_(H) +ΔV are supposed to be defined same as in theembodiment of FIG. 8. In addition, the potential of the positive powersource terminal 48 is supposed to be Ep, the potential of the negativepower source terminal 49 to be Em, the collector-emitter voltage of thenpn type transistor Q1 to be Vcel, the collector-emitter voltage of thepnp type transistor Q2 to be Vce2, and the voltage of the input terminal21a of the ringing preventive circuit 21 to be Vin. The relation of thevoltages Ep, Em, V_(L), V_(H), V_(L) -ΔV, and VH+AV is supposed to beEp>V_(H) +ΔV>V_(H) >V_(L) >V_(L) -ΔV>Em.

When a waveform mixed with noise such as ringing as shown in (a) of FIG.28 is put in the input terminal 21a of the ringing preventive circuit21A, the output potential V_(ri) becomes as follows:

When V_(ri) <V_(L) -ΔV, the output of the L side comparator 31 is Hlevel, and the L side current supply circuit 33A is operated, and acurrent is supplied to the front stage of the buffer amplifier 30. Thecurrent Ip supplied at this time is Ip=(EP-Vin-Vce1)/R2.

When V_(ri) >V_(L) -ΔV, the output of the L side comparator 31 is Llevel, and the L side current supply circuit 33A is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) <V_(H) +ΔV, the output of the H side comparator 32 is Hlevel, and the H side current supply circuit 34A is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) >V_(H) +ΔV, the output of the H side comparator 32 is Llevel, and the H side current supply circuit 34A is operated, and acurrent (negative current) is supplied to the front stage of the bufferamplifier 30. The current Im supplied at this time isIm=(Vin-Em-Vce2)/R3.

In this way, the ringing preventive circuit 21A operates so as to removethe shaded areas A, C in (a) of FIG. 28. At this time, the shaded areasB, D are decreased in oscillation because the damping factor increaseswhen removing the shaded areas A, C, and the peak voltage of theoscillation approaches V_(H) and V_(L). Thus, the waveform of the outputterminal 21b after passing through the ringing preventive circuit 21A isa waveform having range from V_(H) +ΔV to V_(L) -ΔV as shown in FIG. 2.

When this ringing preventive circuit is inserted after the transmissionpath of digital signal, voltage noise such as ringing is prevented, andmalfunction in the circuit succeeding the next stage can be suppressedeffectively.

In the ringing preventive circuit according to the first preferredembodiment, the currents in the condition of R2=R3 and(Ep-Vin-Vce1)=(Vin-Em-Vce2) are in the relation of Ip=Im. If, however,there is a large voltage difference between (Ep-Vin-Vce1) and(Vin-Em-Vce2), or there is a large difference in the level of noise suchas ringing between H and L side, R2 and R3 must be adjusted.

A second embodiment of the ringing preventive circuit of FIG. 8 of theinvention is described below while referring to FIG. 14.

In the ringing preventive circuit of FIG. 14, instead of the npn typebipolar transistor Q1 and pnp type bipolar transistor Q2 used in theringing preventive circuit of FIG. 13, an n channel MOS transistor Q3and a p channel MOS transistor Q4 are used.

In FIG. 14, an L side current supply circuit 33B is composed of theresistance R2 and n channel MOS transistor Q3. The source of thetransistor Q3 is connected to the non-inverting input terminal of theoperational amplifier 50, its drain is connected to the other end of theresistance R2, and the gate is connected to the output terminal of the Lside comparator 31. An H side current supply circuit 34B is composed ofthe resistance R3 and p channel MOS transistor Q4. The source of thetransistor Q4 is connected to the non-inverting input terminal of theoperational amplifier 50, its drain is connected to the other end of theresistance R3, and the gate is connected to the output terminal of the Hside comparator 32.

The operation is described below. The potentials V_(ri), V_(L), V_(H),V_(L) -ΔV, V_(H) +ΔV, Ep, Em are supposed to be defined same as in theembodiment of FIG. 13. The source-drain voltage of the n channeltransistor Q3 is supposed to be Vsd3, the source-drain voltage of the pchannel transistor Q4 to be Vsd4, and the voltage at the input terminal21a of the ringing preventive circuit 21B to be Vin.

When a waveform mixed with noise such as ringing as shown in (a) of FIG.28 is put in the input terminal 21a of the ringing preventive circuit21B, the output side of the ringing preventive circuit 21B is lowered inimpedance by the buffer amplifier 30, and the output voltage of thebuffer amplifier 30 having the gain equal to 1 becomes V_(ri).

When V_(ri) <V_(L) -ΔV, the output of the L side comparator 31 is Hlevel, and the L side current supply circuit 33B is operated, and acurrent is applied to the front stage of the buffer amplifier 30. Thecurrent Ip supplied at this time is Ip=(Ep-Vin-Vsd3)/R2.

When V_(ri) >V_(L) -ΔV, the output of the L side comparator 31 is Llevel, and the L side current supply circuit 33B is not operated, and nocurrent is applied to the front stage of the buffer amplifier 30.

When V_(ri) <V_(H) +ΔV, the output of the H side comparator 32 is Hlevel, and the H side current supply circuit 34B is not operated, and nocurrent is applied to the front stage of the buffer amplifier 30.

When V_(ri) >V_(H) +ΔV, the output of the H side comparator 32 is Llevel, and the H side current supply circuit 34B is operated, and acurrent (negative current) is supplied to the front stage of the bufferamplifier 30. The current Im supplied at this time isIm=(Vin-Em-Vsd4)/R3.

Between the ringing preventive circuit of FIG. 14 and that of FIG. 13,only the type of the transistor used therein is different, and by usingthe ringing preventive circuit of FIG. 14, the same effects as when theringing preventive circuit of FIG. 13 is used are obtained.

A third embodiment of the ringing preventive circuit of FIG. 8 isdescribed by reference to FIG. 15. The difference between the ringingpreventive circuit 21C in FIG. 15 and the ringing preventive circuit 21Ain FIG. 13 lies in that, in order to omit adjustment of the resistanceR2 and R3, the resistance R2 for composing the L side current supplycircuit 33a is changed to a constant current source 51, and that theresistance R3 for composing the H side current supply circuit 35A ischanged to a constant current source 32.

In FIG. 15, the L side current supply circuit 33C is composed of theconstant current source 51 and npn type bipolar transistor Q1. Theconstant current source 51 is connected between the collector of thetransistor Q1 and the positive power source terminal 48. The H sidecurrent supply circuit 34C is composed of the constant current source 52and pnp type bipolar transistor Q2. The constant current source 52 isconnected between the collector of the transistor Q2 and the negativepower source terminal 49.

The operation is described below. The potentials V_(ri), V_(L), V_(H),V_(L) -ΔV, V_(H) +ΔV, Ep, Em are supposed to be defined same as in theembodiment of FIG. 13.

When a waveform mixed with noise such as ringing as shown in (a) of FIG.28 is put in the input terminal 21 of the ringing preventive circuit21C, the output side of the ringing preventive circuit 21C is lowered inimpedance by the buffer amplifier 30, and the output voltage of thebuffer amplifier 30 having the gain equal to 1 becomes V_(ri).

When V_(ri) <V_(L) -ΔV, the output of the L side comparator 31 is Hlevel, and the L side current supply circuit 33C is operated, and aspecific current Ip is supplied to the front stage of the bufferamplifier 30.

When V_(ri) >V_(L) -ΔV, the output of the L side comparator 31 is Llevel, and the L side current supply circuit 33C is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) <V_(H) +ΔV, the output of the H side comparator 32 is Hlevel, and the H side current supply circuit 34C is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) >V_(H) +ΔV, the output of the H side comparator 32 is Llevel, and the H side current supply circuit 34C is operated, and aspecific current (negative current) Im is supplied to the front stage ofthe buffer amplifier 30.

Between the ringing preventive circuit 21C of FIG. 15 and the ringingpreventive circuit 21A of FIG. 13, only the means of flowing a currentto the collector of the transistors Q1, Q2 is different, and by usingthe ringing preventive circuit 21C in FIG. 15, the same effects as whenthe ringing preventive circuit 21A in FIG. 15 is used are obtained. Inthe ringing preventive circuit 21C of FIG. 15, using the constantcurrent sources 51, 52, it is no longer necessary to adjust theresistance values which was necessary in the resistances R2, R3 of theringing preventive circuit 21A in FIG. 13, but since the magnitude ofthe supply current is predetermined, a capacity of the current supplysource may be insufficient in the presence of a large noise such asringing.

A fourth embodiment of the ringing preventive circuit of FIG. 8 of theinvention is described below while referring to FIG. 16.

In the ringing preventive circuit 21D according to FIG. 16, instead ofthe npn type bipolar transistor Q1 and pnp type bipolar transistor Q2used in the ringing preventive circuit 21C of FIG. 15, an n channel MOStransistor Q3 and a p channel MOS transistor Q4 are used.

In FIG. 16, an L side current supply circuit 33D is composed of theconstant current source 51 and n channel MOS transistor Q3. The sourceof the transistor Q3 is connected to the non-inverting input terminal ofthe operational amplifier 50, its drain is connected to the constantcurrent source 51, and the gate is connected to the output terminal ofthe L side comparator 31. An H side inverting supply circuit 34D iscomposed of the constant current source 52 and p channel MOS transistorQ4. The source of the transistor Q4 is connected to the non-invertinginput terminal of the operational amplifier 50, its drain is connectedto the constant current source 52, and the gate is connected to theoutput terminal of the H side comparator 32.

The operation is described below. The potentials V_(ri), V_(L), V_(H),V_(L) -ΔV, V_(H) +ΔV, Ep, Em are supposed to be defined same as in theembodiment of FIG. 15. The source-drain voltage of the n channeltransistor Q3 is supposed to be Vsd3, the source-drain voltage of the pchannel transistor Q4 to be Vsd4, and the voltage at the input terminal21a of the ringing preventive circuit 21D to be Vin.

When a waveform mixed with noise such as ringing as shown in (a) of FIG.28 is put in the input terminal 21a of the ringing preventive circuit21D, the output side of the ringing preventive circuit 21D is lowered inimpedance by the buffer amplifier 30, and the output voltage of thebuffer amplifier 30 of gain 1 becomes V_(ri).

When V_(ri) <V_(L) -ΔV, the output of the L side comparator 31 is Hlevel, and the L side current supply circuit 33D is operated, and aspecific current Ip is supplied to the front stage of the bufferamplifier 30.

When V_(ri) >V_(L) -ΔV, the output of the L side comparator 31 is Llevel, and the L side current supply circuit 33D is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) <V_(H) +ΔV, the output of the H side comparator 32 is Hlevel, and the H side current supply circuit 34D is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) >V_(H) +ΔV, the output of the H side comparator 32 is Llevel, and the H side current supply circuit 34D is operated, and aspecific current (negative current) Im is supplied to the front stage ofthe buffer amplifier 30.

Between the ringing preventive circuit 21D of FIG. 16 and ringingpreventive circuit 21C of FIG. 15, only the type of the transistor usedtherein is different, and by using the ringing preventive circuit 21D ofFIG. 16, the same effects as when the ringing preventive circuit 21C inFIG. 15 is used are obtained.

A fifth embodiment of the ringing preventive circuit of FIG. 8 of theinvention is described below by reference to FIG. 17. In FIG. 17,reference numeral 33E is an L side current supply circuit composed ofnpn type transistor Q5, pnp type transistors Q6 to Q8, and resistancesR4 to R6, 34E is an H side current supply circuit composed of pnp typetransistor Q9, npn type transistors Q10 to Q12, and resistances R7 toR9, and others having the same reference numerals as in FIG. 13 are sameor corresponding parts as in FIG. 13.

The constitution of the L side current supply circuit 33E is described.The transistor Q5 has an emitter connected to the output terminal of theoperational amplifier 50, a base connected to the output terminal of theL side comparator 31, and a collector. The resistance R4 has one sideconnected to the collector of the transistor Q5, and other end. Thetransistor Q6 has a collector connected to the other end of theresistance R4, a base, and an emitter. The resistance R5 has one endconnected to the positive power source terminal 48, and other endconnected to the emitter of the transistor Q6. The transistor Q7 has acollector connected to the non-inverting input terminal of theoperational amplifier 50, an emitter, and a base connected to the baseof the transistor Q6. The resistance R6 has one end connected to thepositive power source terminal 48, and other end connected to theemitter of the transistor Q7. The transistor Q8 has a base connected tothe other end of the resistance R4, an emitter connected to the base ofthe transistor Q6, and a collector connected to the negative powersource terminal 49.

The constitution of the H side current supply circuit 34E is described.The transistor Q9 has an emitter connected to the output terminal of theoperational amplifier 50, a base connected to the output terminal of theH side comparator 32, and a collector. The resistance R7 has one sideconnected to the collector of the transistor Q9, and other end. Thetransistor Q10 has a collector connected to the other end of theresistance R7, a base, and an emitter. The resistance R8 has one endconnected to the negative power source terminal 49, and other endconnected to the emitter of the transistor Q10. The transistor Q11 has acollector connected to the non-inverting input terminal of theoperational amplifier 50, an emitter, and a base connected to the baseof the transistor Q10. The resistance R9 has one end connected to thenegative power source terminal 49, and other end connected to theemitter of the transistor Q11.

The operation is described below. The potentials V_(ri), V_(L), V_(H),V_(L) -ΔV, V_(H) +ΔV, Ep, Em are supposed to be defined same as in theembodiment of FIG. 13.

When a waveform mixed with noise such as ringing as shown in (a) of FIG.28 is put in the input terminal 21 of the ringing preventive circuit21E, the output side of the ringing preventive circuit 21E is lowered inimpedance by the buffer amplifier 30, and the output voltage of thebuffer amplifier 30 having the gain equal to 1 becomes V_(ri).

When V_(ri) <V_(L) -ΔV, the output of the L side comparator 31 is Hlevel, and a current flows into the base of the npn type transistor Q5to turn on the transistor QS. The base-emitter voltage of the transistorQ6 is supposed to be Vbe6, the base-emitter voltage of the transistor Q8to be Vbe8, the base-emitter voltage of the transistor Q7 to be Vbe7,the current flowing in the collector of the transistor Q5 to be Ic5, andthe current flowing in the collector of the transistor Q7 to be It7. Atthis time, the L side current supply circuit 33E supplies the currentIc7 satisfying formula 1 to the front stage of the buffer amplifier 30.

    Ic5≈(Ep-V.sub.ri -Vbe6-Vbe8)/(R4+R5),

    Ic7≈(R5·Ic5+Vbe6-Vbe7)/R6                  Formula 1!

At this time, the greater the noise voltage of ringing or the like, thesmaller becomes the value of V_(ri), and Ic5 becomes larger. As the Ic5becomes larger, Ic7 increases, so that more current is supplied to thefront stage of the buffer amplifier 30.

When V_(ri) >V_(L) -ΔV, the output of the L side comparator 31 is Llevel, and the L side current supply circuit 33E is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) <V_(H) +ΔV, the output of the H side comparator 32 is Hlevel, and the H side current supply circuit 34E is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) >V_(H) +ΔV, the output of the H side comparator 32 is Llevel, and a negative current is supplied to the base of the pnp typetransistor Q9, thereby turning on the transistor Q9. The base-emittervoltage of the transistor Q10 is supposed to be Vbe10, the base-emittervoltage of the transistor Q12 to be Vbe12, the base-emitter voltage ofthe transistor Q11 to be Vbe11, the current flowing in the collector ofthe transistor Q9 to be Ic9, and the current flowing in the collector ofthe transistor Q11 to be Ic11. At this time, the H side current supplycircuit 34 supplies the negative current Ic11 satisfying formula 2 tothe front stage of the buffer amplifier 30.

    Ic9≈(V.sub.ri -Em-Vbe10-Vbe12)/(R7+R8),

    Ic11≈(R8·Ic9+Vbe10-Vbe111)/R9              Formula 2!

At this time, the greater the noise voltage of ringing or the like, thelarger becomes the value of V_(ri), and Ic9 becomes larger. As the Ic9becomes larger, Ic11 increases, so that more negative current issupplied to the front stage of the buffer amplifier 30.

In this way, the ringing preventive circuit 21E operates so as to removethe shaded areas A, C in (a) of FIG. 28. At this time, the shaded areasB,D are decreased in vibration because the damping factor increases whenremoving the shaded areas A, C, and the peak voltage of the vibrationapproaches V_(H) and V_(L). Thus, the waveform of the output terminal21b after passing through the ringing preventive circuit 21E is awaveform limited by V_(H) +ΔV and V_(L) -ΔV as shown in FIG. 2. Whenthis ringing preventive circuit is inserted after the transmission pathof digital signal, voltage noise such as ringing is prevented, andmalfunction in the circuit succeeding the next stage can be suppressedeffectively.

In the invention, moreover, since the current depending on the voltagelevel of noise such as ringing can be supplied to the front stage of thebuffer amplifier 30, a great effect of removing voltage noise is broughtabout.

A sixth embodiment of the ringing preventive circuit of FIG. 8 of theinvention is described below by reference to FIG. 18. In FIG. 17,reference numeral 33F is an L side current supply circuit composed of nchannel MOS transistor Q13, p channel MOS transistors Q14, Q15, andresistances R10 to R12, 34F is an H side current supply circuit composedof p channel MOS transistor Q16, n channel MOS transistors Q17, Q18, andresistances R13 to R15, and others having the same reference numerals asin FIG. 11 are same or corresponding parts as in FIG. 11.

The operation is described below. The potentials V_(ri), V_(L), V_(H),V_(L) -ΔV_(H) +ΔV, Ep, Em are supposed to be defined same as in FIG. 13.When a waveform mixed with noise such as ringing as shown in (a) of FIG.28 is put in the input terminal 21a of the ringing preventive circuit21F, the output side of the ringing preventive circuit 21F is lowered inimpedance by the buffer amplifier 30, and the output voltage of thebuffer amplifier 30 of gain 1 becomes V_(ri).

When V_(ri) <V_(L) -ΔV, the output of the L side comparator 31 is Hlevel, and the n channel MOS transistor Q13 is turned on. Thedrain-source voltage of the transistor Q13 is supposed to be Vds13, thegate-source voltage of the transistor Q4 to be Vgs14, the gate-sourcevoltage of the transistor Q15 to be Vgs15, the current flowing in thedrain of the transistor Q13 to be Id13, and the current flowing in thedrain of the transistor Q15 to be Id15. At this time, the L side currentsupply circuit 33F supplies the current Id15 satisfying formula 3 to thefront stage of the buffer amplifier 30.

    Id13≈(Ep-V.sub.ri -Vgs14-Vds13)/(R10+R11),

    Id15≈(R11·Id13+Vgs14-Vgs15)/R12            Formula 3!

At this time, the greater the noise voltage of ringing or the like, thesmaller becomes the value of V_(ri), and Id13 becomes larger. As theId13 becomes larger, Id15 increases, so that more current is supplied tothe front stage of the buffer amplifier 30.

When V_(ri) >V_(L) -ΔV, the output of the L side comparator 31 is Llevel, and the L side current supply circuit 33F is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) <V_(H) +ΔV, the output of the H side comparator 32 is Hlevel, and the H side current supply circuit 34F is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) >V_(H) +ΔV, the output of the H side comparator 32 is Llevel, and the p channel MOS transistor Q16 is turned on. Thesource-drain voltage of the transistor Q16 is supposed to be Vsd16, thegate-source voltage of the transistor Q17 to be Vgs17, the gate-sourcevoltage of the transistor Q18 to be Vgs18, the current flowing in thedrain of the transistor Q16 to be Id16, and the current flowing in thedrain of the transistor Q18 to be Id18. At this time, the H side currentsupply circuit 34F supplies the negative current Id18 satisfying formula4 to the front stage of the buffer amplifier 30.

    Id16≈(V.sub.ri -Em-Vgs17-Vsd16)/(R13+R14),

    Id18≈(R14·Id16+Vgs17-Vsd18)/R15            Formula 4!

At this time, the greater the noise voltage of ringing or the like, thelarger becomes the value of V_(ri), and Id16 becomes larger. As the Id16becomes larger, Id18 increases, so that more negative current issupplied to the front stage of the buffer amplifier 30.

In this way, the ringing preventive circuit 21F operates so as to removethe shaded areas A, C and shaded areas B, D in (a) of FIG. 28, and thewaveform at the output terminal 21b of the ringing preventive circuit21F becomes as shown in FIG. 2.

By using the ringing preventive circuit of the sixth embodiment, thesame effects as using the ringing preventive circuit shown in the fifthpreferred embodiment are obtained.

A seventh embodiment of the ringing preventive circuit of FIG. 8 of theinvention is described by referring to FIG. 19. In the ringingpreventive circuit according to FIGS. 17 and 18, the waveform put intothe input terminal 21a of the ringing preventive circuits 21E, 21F maynot be constant in V_(H) -V_(L) or may have a difference betweenEP-V_(L) and V_(H) -EM. For example, in the ringing preventive circuit21F in FIG. 18, at this time, there is a great fluctuation between thecurrents Id15 and Id18 supplied to the front stage of the bufferamplifier 30, and hence it is to adjust the value of the resistors R10to R15 or the gate length or gate width of transistors Q14, Q15, Q17,Q18 by the waveform put into the input terminal 21a of the ringingpreventive circuit 21F. The ringing preventive circuit in FIG. 19 isdesigned so that such adjustment is not necessary. What differs betweenthe ringing preventive circuit in FIG. 17 and the ringing preventivecircuit in FIG. 19 is that the ringing preventive circuit of FIG. 19 hasan immediate voltage generating circuit 55 composed of resistances R23,R24, and the emitters of the transistors Q20 and Q24 are connected tothe intermediate voltage Vm.

In FIG. 19, reference numeral 33G is an L side current supply circuitcomposed of npn type transistor Q20, pnp type transistors Q21 to Q23,and resistances R16 to R18, 34G is an H side current supply circuitcomposed of pnp type transistor Q24, npn type transistors Q25 to Q27,and resistances R20 to R22, and others of same reference numerals as inFIG. 13 are same or corresponding parts as in FIG. 13. The diode D20composed of resistance R19 and pnp type transistor is intended toprevent malfunction of the ringing preventive circuit 21G at lowtemperature.

First, the constitution of the L side current supply circuit 33G isdescribed. The transistor Q20 has an emitter provided with anintermediate voltage Vm from an intermediate voltage generating circuit55, a base connected to the output terminal of the L side comparator 31,and a collector. The resistance R16 has one end connected to thecollector of the transistor Q20, and other end. The transistor Q21 has acollector connected to the other end of the resistance R16, a base, andan emitter. The resistance R17 has one end connected to a positive powersource terminal 48, and other end connected to the emitter of thetransistor Q21. The transistor Q23 has a base connected to the other endof the resistance R16, a collector connected to a negative power sourceterminal 49, and an emitter connected to the base of the transistor Q21.The transistor Q22 has a base connected to the base of the transistorQ21, and an emitter. The resistance R18 has one end connected to thepositive power source terminal 48, and other end connected to theemitter of the transistor Q22.

Incidentally, one end of the resistance R19 is connected to the positivepower source terminal, and the other end of the resistance R19 isconnected to the anode of the diode D20. The cathode of the diode D20 isconnected to the emitter of the transistor Q23.

Next, the constitution of the H side current supply circuit 34G isdescribed. The transistor Q24 has an emitter provided with anintermediate voltage Vm from the intermediate generating circuit 55, abase connected to the output terminal of the H side comparator 32, and acollector. The resistance R20 has one end connected to the collector ofthe transistor Q24, and other end. The transistor Q25 has a collectorconnected to the other end of the resistance R20, a base, and anemitter. The resistance R21 has one end connected to the emitter of thetransistor Q25 and the other end connected to the negative power sourceterminal 49. The transistor Q27 has an emitter connected to the base ofthe transistor Q25, a base connected to the collector of the transistorQ25, and a collector connected to the positive power source terminal 48.The transistor Q26 has a base connected to the base of the transistorQ25, a collector connected to the non-inverting input terminal of theoperational amplifier 50, and an emitter. The resistance R22 has one endconnected to the negative power source terminal 49, and other endconnected to the emitter of the transistor Q26.

The intermediate voltage generating circuit 55 is composed of theresistance R23 having one end connected to the positive power sourceterminal 48, and other end connected to the node M for generating theintermediate voltage Vm, and the resistance R24 having one end connectedto the negative power source terminal 49 and other end connected to thenode M for generating an intermediate voltage.

The operation is described below. The potentials V_(ri), V_(L), V_(H),V_(L) -ΔV, V_(H) +ΔV, Ep, Em are supposed to be defined same as in theembodiment of FIG. 13.

When a waveform mixed with noise such as ringing as shown in (a) of FIG.28 is put in the input terminal 21a of the ringing preventive circuit21G, the output side of the ringing preventive circuit 21G is lowered inimpedance by the buffer amplifier 30, and the output voltage of thebuffer amplifier 30 having the gain equal to 1 becomes V_(ri).

When V_(ri) <V_(L) -ΔV, the output of the L side comparator 31 is Hlevel, and a current flows into the base of the npn type transistor Q20to turn on the transistor Q20. The base-emitter voltage of thetransistor Q21 is supposed to be Vbe21, the base-emitter voltage of thetransistor Q23 to be Vbe23, the base-emitter voltage of the transistorQ22 to be Vbe22, the current flowing in the collector of the transistorQ20 to be Ic20, and the current flowing in the collector of thetransistor Q22 to be Ic22. At this time, the L side current supplycircuit 33G supplies the current Ic22 satisfying formula 5 to the frontstage of the buffer amplifier 30.

    Ic20≈(Ep-Vm-Vbe21-Vbe23)/(R16+R17),

    Ic22≈(R17·Ic20+Vbe21-Vbe22)/R18            Formula 5!

When V_(ri) >V_(L) -ΔV, the output of the L side comparator 31 is Llevel, and the L side current supply circuit 33G is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) <V_(H) +ΔV, the output of the H side comparator 32 is Hlevel, and the H side current supply circuit 34G is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) >V_(H) +ΔV, the output of the H side comparator 32 is Llevel, and a negative current is supplied to the base of the pnp typetransistor Q24, thereby turning on the transistor Q24. The base-emittervoltage of the transistor Q25 is supposed to be Vbe25, the base-emittervoltage of the transistor Q27 to be Vbe27, the base-emitter voltage ofthe transistor Q26 to be Vbe26, the current flowing in the collector ofthe transistor Q24 to be Ic24, and the current flowing in the collectorof the transistor Q26 to be Ic26. At this time, the H side currentsupply circuit 34G supplies the negative current Ic26 satisfying formula6 to the front stage of the buffer amplifier 30.

    Ic24≈(Vm-Em-Vbe25-Vbe27)/(R20+R21),

    Ic26≈(R21·Ic24+Vbe25-Vbe26)/R22.           Formula 6!

In this way, the ringing preventive circuit 21G operates so as to removethe shaded areas A, C in (a) of FIG. 28. At this time, the shaded areasB, D are decreased in oscillation because the damping factor increaseswhen removing the shaded areas A, C, and the peak voltage of theoscillation approaches V_(H) and V_(L). Thus, the waveform of the outputterminal 21b after passing through the ringing preventive circuit 21G isa waveform having a range from V_(H) +ΔV to V_(L) -ΔV as shown in FIG.2.

When this ringing preventive circuit is inserted after the transmissionpath of digital waveform, voltage noise such as ringing is prevented,and malfunction in the circuit succeeding the next stage can besuppressed as far as possible.

In the ringing preventive circuit 21G in FIG. 19, when the voltageV_(ri) of the waveform put into the input terminal 21a is in therelation of Em+ΔV<V_(ri) <Ep-ΔV, constant currents Ic22, Ic26 may bealways supplied to the front stage of the buffer amplifier 30.

An eighth embodiment of the ringing preventive circuit of FIG. 8 of theinvention is described by reference to FIG. 20. In the ringingpreventive circuit of FIG. 20, instead of the bipolar transistor Q20 toQ27 used in the ringing preventive circuit 21G in FIG. 19, MOStransistors Q30 to Q35 are used.

That is, the ringing preventive circuit 21H shown in FIG. 20 is to solvethe problem of the ringing preventive circuit 21F composed of CMOS shownin FIG. 18. What the ringing preventive circuit 21H in FIG. 20 differsfrom the ringing preventive circuit 21F in FIG. 18 is that anintermediate voltage generating circuit composed of two resistance isprovided, and that the sources of the transistors of the H side and Lside current supply circuits controlled by the comparators are connectedto the intermediate voltage Vm.

In FIG. 20, reference numeral 33H is an L side current supply circuitcomposed of n channel MOS transistor Q30, p channel MOS transistors Q31,Q32, and resistances R25 to R27, 34H is an H side current supply circuitcomposed of p channel MOS transistor Q33, n channel MOS transistors Q34,Q35, and resistances R28 to R30, 55 is an intermediate voltagegenerating circuit composed of resistances R23, R24, and other referencenumerals same as in FIG. 13 are same or corresponding parts as in FIG.13.

First, the constitution of the L side current supply circuit 33H isdescribed. The transistor Q30 has a source provided with an intermediatevoltage Vm from the intermediate voltage generating circuit 55, a gateconnected to the output terminal of the L side comparator 31, and adrain. The resistance R25 has one end connected to the drain of thetransistor Q30, and other end. The transistor Q31 has a drain connectedto the other end of the resistance R25, a gate connected to the otherend of the resistance R25, and a source. The resistance R26 has one endconnected to a positive power source terminal 48, and other endconnected to the source of the transistor Q31. The transistor Q32 has agate connected to the gate of the transistor Q31, a drain connected tothe non-inverting input terminal of the operational amplifier 50, and asource. The resistance R27 has one end connected to the positive powersource terminal 48, and other end connected to the source of thetransistor Q32.

Next, the constitution of the H side current supply circuit 34H isdescribed. The transistor Q33 has a source provided with an intermediatevoltage Vm from the intermediate voltage generating circuit 55, a gateconnected to the output terminal of the H side comparator 32, and adrain. The resistance R28 has one end connected to the drain of thetransistor Q33, and other end. The transistor Q34 has a drain connectedto the other end of the resistance R28, a gate connected to the otherend of the resistance R28, and a source. The resistance R29 has one endconnected to a negative power source terminal 49, and other endconnected to the source of the transistor Q34. The transistor Q35 has agate connected to the gate of the transistor Q34, a drain connected tothe non-inverting input terminal of the operational amplifier 50, and asource. The resistance R30 has one end connected to the source of thetransistor Q35, and other end connected to the negative power sourceterminal 49.

The intermediate voltage generating circuit 55 is composed of theresistance R23 having one end connected to the positive power sourceterminal 48, and other end connected to the node M for generating theintermediate voltage Vm, and the resistance R24 having one end connectedto the negative power source terminal 49, and other end connected to thenode M for generating the intermediate voltage.

The operation is described below. The potentials V_(ri), V_(L), V_(H),V_(L) -ΔV, V_(H) +ΔV, Ep, Em are supposed to be defined same as in theembodiment of FIG. 13.

When a waveform mixed with noise such as ringing as shown in (a) of FIG.28 is put in the input terminal 21 of the ringing preventive circuit21H, the output side of the ringing preventive circuit 21H is lowered inimpedance by the buffer amplifier 30, and the output voltage of thebuffer amplifier 30 having the gain equal to 1 becomes V_(ri).

When V_(ri) <V_(L) -ΔV, the output of the L side comparator 31 is Hlevel, and a current flows into the base of the n channel MOS transistorQ30 to turn on the transistor Q30. The drain-source voltage of thetransistor Q30 is supposed to be Vds30, the gate-source voltage of thetransistor Q31 to be Vgs31, the gate-source voltage of the transistorQ32 to be Vgs32, the current flowing in the drain of the transistor Q30to be Id30, and the current flowing in the drain of the transistor Q32to be Id32. At this time, the L side current supply circuit 33H suppliesthe current Id32 satisfying formula 7 to the front stage of the bufferamplifier 30.

    Id30≈(Ep-Vm-Vgs31-Vds30)/(R25+R26),

    Id32≈(R26·Id30+Vgs31 Vgs32)/R27            Formula 7!

When V_(ri) >V_(L) -ΔV, the output of the L side comparator 31 is Llevel, and the L side current supply circuit 33H is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) <V_(H) +ΔV, the output of the H side comparator 32 is Hlevel, and the H side current supply circuit 34H is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) >V_(H) +ΔV, the output of the H side comparator 32 is Llevel, and the p channel MOS transistor Q33 is turned on. Thesource-drain voltage of the transistor Q33 is supposed to be Vsd33, thegate-source voltage of the transistor Q34 to be Vgs34, the gate-sourcevoltage of the transistor Q35 to be Vgs35, the current flowing in thedrain of the transistor Q33 to be Id33, and the current flowing in thedrain of the transistor Q35 to be Id35. At this time, the H side currentsupply circuit 34 supplies the negative current Id35 satisfying formula8 to the front stage of the buffer amplifier 30.

    Id33≈(V.sub.ri -Em-Vgs34-Vsd33)/(R28+R29),

    Id35≈(R29·Id33+Vgs34-Vsd35)/R30            Formula 8!

In this way, the ringing preventive circuit 21H operates so as to removethe shaded areas A, C in (a) of FIG. 28. At this time, the shaded areasB, D are decreased in oscillation because the damping factor increaseswhen removing the shaded areas A, C, and the peak voltage of theoscillation approaches V_(H) and V_(L). Thus, the waveform of the outputterminal 21b after passing through the ringing preventive circuit 21H isa waveform having a range from V_(H) +ΔV to V_(L) -ΔV as shown in FIG.2.

When this ringing preventive circuit is inserted after the transmissionpath of digital signal, voltage noise such as ringing is prevented, andmalfunction in the circuit succeeding the next stage can be suppressedeffectively.

Using the ringing preventive circuit 21H of FIG. 20, the same effects aswhen using the ringing preventive circuit 21G in FIG. 19 are obtained.The ringing preventive circuit 21 can always supply constant currentsId32, Id35 to the front stage of the buffer amplifier 30 as far as thevoltage V_(ri) of the waveform applied to the input terminal 21a is inthe relation of Em+ΔV<V_(ri) <Ep-ΔV.

A ninth embodiment of the ringing preventive circuit of FIG. 8 of theinvention is described below by reference to FIG. 21. The intermediatevoltage generating circuit 55 of the ringing preventive circuit 21G ofFIG. 19 was composed of divided resistances, and a current flows intothe intermediate portion M, and the intermediate voltage Vm is not equalto (Ep-Em)/2, but the currents Ic22, Ic26 supplied in the front stage ofthe buffer amplifier 30 decrease. The ringing preventive circuit 21Jshown in FIG. 21 is to solve the problem of the ringing preventivecircuit 21G of FIG. 19. What the ringing preventive circuit 21J of FIG.21 differs from the ringing preventive circuit 21G shown in FIG. 19 isthat the intermediate voltage generating circuit is changed from theresistance division system to push-pull system in order to obtain astable intermediate potential.

In FIG. 21, reference numeral 56 is an intermediate voltage generatingcircuit composed of an npn bipolar transistor Q28, a pnp bipolartransistor Q29, a constant current power source 57, and diodes D21 toD23, and other reference numerals same as in FIG. 19 denote same orcorresponding parts as in FIG 19. The diode D20 composed of resistanceR19 and pnp transistor is intended to prevent malfunction of the ringingpreventive circuit 21 at low temperature. The diodes D21 and D23 arecomposed of, for example, npn type transistors. The diode D22 iscomposed of pnp type transistor.

The difference between the ringing preventive circuit shown in FIG. 19and the ringing preventive circuit shown in FIG. 21 lies only in thedifference of constitution of the intermediate voltage generatingcircuit, and hence only the intermediate voltage generating circuit isdescribed below. In the intermediate voltage generating circuit 56 withpush-pull amplifier, the constant current power source 57 is connectedto a positive power source terminal 48. The transistor Q28 has acollector connected to the positive power source terminal 48, an emitterconnected to the node M for producing an intermediate voltage Vm, and abase connected to the constant current power source 57. The transistorQ29 has a collector connected to a negative power source terminal 49, anemitter connected to the node M, and a base. The diode D21 has an anodeconnected to the constant current power source 57, and a cathode. Thediode D22 has an anode connected to the cathode of the diode D21, and acathode connected to the base of the transistor Q29. The diode D23 iscomposed of plural diodes connected in series between the negative powersource terminal 49 and the base of the transistor Q29, and a biasvoltage is applied to all diodes in forward direction. The action upononset of ringing is same as in the ringing preventive circuit 21G shownin FIG. 19.

Using the ringing preventive circuit 21J of FIG. 21, the same effects aswhen using the ringing preventive circuit 21G shown in FIG. 19 areobtained. Using the ringing preventive circuit 21J of FIG. 21, since theintermediate voltage generating circuit56 is structured in push-pullsystem, the intermediate voltage Vm hardly changes, and stable currentsIc22, Ic26 can be supplied to the front stage of the buffer amplifier30.

In this preferred embodiment, the intermediate voltage generatingcircuit 56 has a push-pull amplifier, but same effects are obtained inthe circuit composition by making use of the band gap of thesemiconductor in order to obtain a stabler intermediate potentialregardless of temperature changes.

A tenth embodiment of the ringing preventive circuit of FIG. 8 of theinvention is described below by reference to FIG. 22. The intermediatevoltage generating circuit 55 of the ringing preventive circuit 21H ofthe preferred embodiment of FIG. 20 was composed of divided resistances,and a current flows into the intermediate portion M, and theintermediate voltage Vm is not equal to (Ep-Em)/2, but the currentsId32, Id35 supplied in the front stage of the buffer amplifier 30decrease. The ringing preventive circuit 21K shown in FIG. 22 is tosolve the problem of the ringing preventive circuit 21H of FIG. 20. Whatthe ringing preventive circuit 21K of FIG. 22 differs from the ringingpreventive circuit 21H shown in FIG. 20 is that basic component of theintermediate voltage generating circuit is a push-pull amplifier insteadof the voltage divider with resistors in order to obtain a stableintermediate potential.

In FIG. 22, reference numeral 58 is an intermediate voltage generatingcircuit of push-pull amplifier composed of n channel MOS transistorsQ40, Q42, Q44, p channel MOS transistors Q41, Q43, and a constantcurrent power source 59, and other reference numerals same as in FIG. 20denote same or corresponding parts as in FIG. 20. To keep stability ofthe intermediate potential regardless of temperature changes, aresistance may be used instead of the transistor Q44.

The action upon onset of ringing is same as in the ringing preventivecircuit 21H shown in FIG. 20.

Since the intermediate generating circuit 58 of the ringing preventivecircuit 21K of FIG. 22 is structured in push-pull system, theintermediate voltage Vm hardly changes, and stable currents Id32, Id35can be supplied to the front stage of the buffer amplifier 30. In thispreferred embodiment, the intermediate voltage generating circuit 58 isof push-pull system, but same effects are obtained in the circuitcomposition by making use of the band gap of the semiconductor in orderto obtain a stabler intermediate potential regardless of temperaturechanges. An eleventh embodiment of the ringing preventive circuit ofFIG. 8 of the invention is described below by reference to FIG. 23. Inthe ringing preventive circuit 21J of FIG. 21, if a waveform of a largenoise voltage due to ringing or the like is put into the input terminal21a of the ringing preventive circuit 21J, the noise of ringing or thelike may not be sufficiently prevented by predetermined constantcurrents Ic22, Ic26. The ringing preventive circuit 21L shown in FIG. 23is intended to solve the problem of the ringing preventive circuit 21Jof FIG. 21. What the ringing preventive circuit 21L of FIG. 23 differsfrom the ringing preventive circuit 21J of FIG. 21 is that differentialvoltage amplifiers are added for amplifying the noise voltage in orderto supply a current corresponding to the noise voltage of ringing or thelike into the front stage of the buffer amplifier 30.

In FIG. 23, reference numeral 33L is an L side current supply circuitcomposed of an npn type transistor Q50, pnp type transistors Q21 to Q23,and resistances R16 to R19, 34L is an H side current supply circuitcomposed of a pnp type transistor Q51, npn type transistors Q25 to Q27,and resistances R25 to R27, 62 is a differential amplifying circuit foramplifying the differential voltages of the reference power sources E3,E4 and the output of the operational amplifier 50 individually, andoutputting the intermediate voltage Vm to the L side current supplycircuit 33L and H side current supply circuit 34L as the reference, andother reference numerals same as in FIG. 21 are same or correspondingparts as in FIG. 21.

The difference in connection relation of circuits between the L sidecurrent supply circuit 33G shown in FIG. 21 and the L side currentsupply circuit 33L in FIG. 23 lies only in the connection of thetransistor Q20 and transistor Q50. The emitter of the transistor Q20 isconnected to the node M of the intermediate voltage generating circuit56, whereas the emitter of the transistor Q50 is connected to the outputterminal of the operational amplifier 63.

Likewise, the difference in connection relation of circuits between theH side current supply circuit 34G shown in FIG. 21 and the H sidecurrent supply circuit 34L in FIG. 23 lies only in the connection of thetransistor Q24 and transistor Q51. The emitter of the transistor Q24 isconnected to the node M of the intermediate voltage generating circuit56, whereas the emitter of the transistor Q51 is connected to the outputterminal of the operational amplifier 64.

The constitution of the differential amplifier 62 is described below.The operational amplifier 63 has its non-inverting input terminalconnected to the node M for generating the intermediate voltage of theintermediate voltage generating circuit 56 through the resistance R43and also connected to the output terminal of the operational amplifier50 through the resistance 47, its inverting input terminal connected tothe positive electrode of the reference power source E3 through theresistance R45, and its output terminal connected to the emitter of thetransistor Q50, and its output terminal and non-inverting input terminalare connected through the resistance R41, and the output is suppliedback. An operational amplifier 64 has its non-inverting input terminalconnected to the node M for generating the intermediate voltage of theintermediate voltage generating circuit 56 through the resistance R44and also connected to the positive electrode of the reference powersource E4 through the resistance R48, its inverting input terminalconnected to the output terminal of the operational amplifier 50 throughthe resistance R46, and its output terminal connected to the emitter ofthe transistor Q51, and its output terminal and non-inverting inputterminal are connected through the resistance R42, and the output issupplied back. The reference power source E3 generates the L sidereference voltage V_(L), and the reference power source E4 generates theH side reference voltage V_(H). The relation of the resistance values ofthe resistances R41 to R48 is R45=R47=R46=R48=Rx, R41=R43=R42=R44=Ry.The diodes D20 composed of resistance R19 and pnp type transistor isintended to prevent malfunction at low temperature.

The operation is described below. The potentials V_(ri), V_(L), V_(H),V_(L) -ΔV, V_(H) +ΔV, Ep, Em are supposed to be defined same as in theembodiment of FIG. 13, except that Vm=(Ep-Em)/2+Em.

When V_(ri) <V_(L) -ΔV, the output of the L side comparator 31 is Hlevel, and a current flows into the base of the npn type transistor Q50to turn on the transistor Q50. The base-emitter voltage of thetransistor Q21 is supposed to be Vbe21, the base-emitter voltage of thetransistor Q23 to be Vbe23, the base-emitter voltage of the transistorQ22 to be Vbe22, the current flowing in the collector of the transistorQ50 to be Ic50, the current flowing in the collector of the transistorQ22 to be Ic22, and the output voltage of the operational amplifier 63to be VL0. At this time, the L side current supply circuit 33L suppliesthe current Ic22 satisfying formula 9 to the front stage of the bufferamplifier 30.

    VL0=Vm-(Ry/Rx)·(E3-V.sub.ri),

    Ic50≈(Ep-VL0-Vbe21-Vbe23)/(R16+R17),

    Ic22≈(R17·Ic50+Vbe21-Vbe22)/R18            Formula 9!

When V_(ri) >V_(L) ΔV, the output of the L side comparator 31 is Llevel, and the L side current supply circuit 33L is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) <V_(H) +ΔV, the output of the H side comparator 32 is Hlevel, and the H side current supply circuit 34L is not operated, and nocurrent is supplied to the front stage of the buffer amplifier 30.

When V_(ri) >V_(H) +ΔV, the output of the H side comparator 32 is Llevel, and a negative current is supplied to the base of the pnp typetransistor Q51, thereby turning on the transistor Q51. The base-emittervoltage of the transistor Q25 is supposed to be Vbe25, the base-emittervoltage of the transistor Q27 to be Vbe27, the base-emitter voltage ofthe transistor Q26 to be Vbe26, the current flowing in the collector ofthe transistor Q51 to be Ic51, the current flowing in the collector ofthe transistor Q26 to be Ic26, and the output voltage of the operationalamplifier 64 to be VH0. At this time, the H side current supply circuit34L supplies the negative current Ic26 satisfying formula 10 to thefront stage of the buffer amplifier 30.

    VH0=Vm+(Ry/Rx)·(V.sub.ri -E4),

    Ic51≈(Vm-VH0-Vbe25-Vbe27)/(R20+R21),

    Ic26≈(R21·Ic51+Vbe25-Vbe26)/R22            Formula 10!

In this way, the ringing preventive circuit 21L operates so as to removethe shaded areas A, C in (a) of FIG. 28. At this time, the shaded areasB, D are decreased in oscillation because the damping factor increaseswhen removing the shaded areas A, C, and the peak voltage of theoscillation approaches V_(H) and V_(L). Thus, the waveform of the outputterminal 21b after passing through the ringing preventive circuit 21L isa waveform having a range from V_(H) +ΔV to V_(L) -ΔV as shown in FIG.2.

When this ringing preventive circuit 21L is inserted after thetransmission path of digital signal, voltage noise such as ringing isprevented, and malfunction in the circuit succeeding the next stage canbe suppressed effectively.

In the ringing preventive circuit 21L of FIG. 23, since the currentsIc22, Ic26 depending on the voltage level of noise such as ringing canbe supplied to the front stage of the buffer amplifier 30, ringing andother noise can be prevented without having to adjust the values of theresistances R16 to R22 or emitter area of the transistors Q21, Q22, Q25,Q26, for all input waveforms in the range of Em+ΔV<V_(ri) <Ep-ΔV.

A twelfth embodiment of the ringing preventive circuit of FIG. 8 of theinvention is described below by reference to FIG. 24. In the ringingpreventive circuit 21K of FIG. 22, if a waveform of a large noisevoltage due to ringing or the like is put into the input terminal 21a ofthe ringing preventive circuit 21K, the noise of ringing or the like maynot be sufficiently prevented by predetermined constant currents Id32,Id35. The ringing preventive circuit 21N of FIG. 24 is intended to solvethe problem of the ringing preventive circuit 21K of FIG. 22. What theringing preventive circuit 21N of FIG. 24 differs from the ringingpreventive circuit 21K of FIG. 22 is that differential voltageamplifiers are added for amplifying the noise voltage in order to supplya current corresponding to the noise voltage of ringing or the like intothe front stage of the buffer amplifier 30.

In FIG. 24, reference numeral 33N is an L side current supply circuitcomposed of an n channel MOS transistor Q60, p channel MOS transistorsQ31, Q32, and resistances R25 to R27, 34N is an H side current supplycircuit composed of a p channel MOS transistor Q61, n channel MOStransistors Q34, Q35, and resistances R29, R30, 62 is a differentialamplifying circuit for amplifying the differential voltages of thereference power sources E3, E4 and the output of the operationalamplifier 50 individually, and outputting the intermediate voltage Vm tothe L side current supply circuit 33N and H side current supply circuit34N as the reference, and other reference numerals same as in FIG. 22are same or corresponding parts as in FIG. 22.

The difference in connection relation of circuits between the L sidecurrent supply circuit 33H shown in FIG. 22 and the L side currentsupply circuit 33N in FIG. 24 lies only in the connection of thetransistor Q30 and transistor Q60. The emitter of the transistor Q30 isconnected to the node M of the intermediate voltage generating circuit58, whereas the emitter of the transistor Q56 is connected to the outputterminal of the operational amplifier 63.

Likewise, the difference in connection relation of circuits between theH side current supply circuit 34H shown in FIG. 22 and the H sidecurrent supply circuit 34N in FIG. 24 lies only in the connection of thetransistor Q33 and transistor Q61. The emitter of the transistor Q33 isconnected to the node M of the intermediate voltage generating circuit58, whereas the emitter of the transistor Q61 is connected to the outputterminal of the operational amplifier 64.

The constitution of the differential amplifier 62 is same as that inFIG. 22, except that the output terminal of the operational amplifier 63is connected to the drain of the transistor Q60, and that the outputterminal of the operational amplifier 64 is connected to the drain ofthe transistor Q51.

The operation is described below. The potentials V_(ri), V_(L), V_(H),V_(L) -ΔV, V_(H) +ΔV, Ep, Em are supposed to be defined same as in theembodiment of FIG. 13, except that Vm=(Ep-Em)/2+Em.

When V_(ri) <V_(L) -ΔAV, the output of the L side comparator 31 is Hlevel, and the n channel MOS transistor Q60 is turned on. Thedrain-source voltage of the transistor Q60 is supposed to be Vds60, thegate-source voltage of the transistor Q31 to be Vgs31, the gate-sourcevoltage of the transistor Q32 to be Vgs32, the current flowing in thedrain of the transistor Q60 to be Id60, the current flowing in the drainof the transistor Q32 to be Id32, and the output voltage of theoperational amplifier 63 to be VL0. At this time, the L side currentsupply circuit 33N supplies the current Id32 satisfying formula 11 tothe front stage of the buffer amplifier 30.

    VL0=Vm-(Ry/Rx)·(E3-V.sub.ri),

    Id60≈(Ep-VL0-Vgs31-Vds60)/(R25+R26),

    Id32≈(R26·Id60+Vgs31-Vgs32)/R27            Formula 11!

When V_(ri) >V_(L) -ΔV, the output of the L side comparator 31 is Llevel, and the L side current supply circuit 33N is not operated, and nocurrent is applied to the front stage of the buffer amplifier 30.

When V_(ri) <V_(H) +ΔV, the output of the H side comparator 32 is Hlevel, and the H side current supply circuit 34N is not operated, and nocurrent is applied to the front stage of the buffer amplifier 30.

When V_(ri) >V_(H) +ΔV, the output of the H side comparator 32 is Llevel, and the p channel MOS transistor Q61 is turned on. Thesource-drain voltage of the transistor Q61 is supposed to be Vsd61, thegate-source voltage of the transistor Q34 to be Vgs34, the gate-sourcevoltage of the transistor Q35 to be Vgs35, the current flowing in thedrain of the transistor Q61 to be Id61, the current flowing in the drainof the transistor Q35 to be Id35, and the output voltage of theoperational amplifier 64 to be VH0. At this time, the H side currentsupply circuit 34N applies the negative current Id35 satisfying formula12 to the front stage of the buffer amplifier 30.

    VH0=Vm+(Ry/Rx)·(V.sub.ri -E4),

    Id61≈(VH0-Em-Vgs34-Vsd61)/(R28+R29),

    Id35≈(R29·Id61+Vgs34-Vsd35)/R30            Formula 12!

In this way, same as in FIG. 23, the ringing preventive circuit 21Noperates so as to remove the shaded areas A to D in FIG. 28. When thisringing preventive circuit 21N is inserted after the transmission pathof digital signal, voltage noise such as ringing is prevented, andmalfunction in the circuit succeeding the next stage can be suppressedeffectively.

In the ringing preventive circuit 21N of FIG. 24, since the currentsId32, Id35 depending on the voltage level of noise such as ringing canbe supplied to the front stage of the buffer amplifier 30, ringing andother noise can be prevented without having to adjust the value of theresistances R25 to R30 or gate length or gate width of the transistorsQ31, Q32, Q34, Q35, for all input waveforms in the range of Em+ΔV<V_(ri)<Ep-ΔV.

In the ringing preventive circuits of the first and subsequent preferredembodiments of the circuit of FIG. 8, the operational amplifier isconnected to the voltage follower as an example of the buffer amplifier30, but it may be connected to the emitter follower or other bufferamplifier, and same effects as in the foregoing preferred embodimentsare obtained.

The reference power sources E1 to E4 are connected to the groundingvoltage GND, but they may be also based on base line of pulse or othervalues relating to the direct-current component of a pulse wave, andsame effects are in the preferred embodiments are obtained. Generationof direct-current component of a pulse wave is easily realized from theprior art.

In the description of the preferred embodiments, the range of theoperating voltage of the ringing preventive circuit is not specificallyexplained, but as shown in FIG. 30, briefly, the time tr when thecurrent starts to be supplied from the ringing preventive circuit issomewhere from the time t_(t) when the output waveform crosses V_(H) +ΔVtill the time tp when the voltage of the waveform reaches the maximum.That is, the following relation is established.

    t.sub.t ≦t.sub.r <<t.sub.p                           Formula 13!

The effect of preventing ringing is greater as the reaction time (t_(r)-t_(t)) of the ringing preventive circuit is closer to 0, and as ΔV iscloser to 0. FIG. 2 is a diagram showing the ideal characteristic inwhich the reaction time of the ringing preventive circuit is infinitelyclose to 0. Taking this point into consideration, the output waveform asshown in FIG. 31 is obtained.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

I claim:
 1. A ringing preventive circuit comprising:buffer amplifierhaving an input terminal and an output terminal; a first comparatorhaving a first input terminal connected to said output terminal of saidbuffer amplifier, a second input terminal provided with a firstreference voltage, and an output terminal, for outputting a firstcontrol signal from said output terminal in response to a result ofcomparison between voltages at said first and second input terminals;and a first current supply circuit having a first terminal connected toa first power source for supplying a first voltage, a second terminalconnected to said input terminal of said buffer amplifier, and a controlsignal input terminal connected to said output terminal of firstcomparator, for controlling conduction and non-conduction of a currentflowing from said first terminal to said second terminal as a functionof said first control signal.
 2. The ringing preventive circuit of claim1, wherein said buffer amplifier includes a voltage follower operationalbuffer amplifier comprising an output stage and a voltage follower in aninput stage.
 3. The ringing preventive circuit of claim 2, wherein theoutput stage of said voltage follower operational buffer amplifierincludes a totem-pole amplifier.
 4. The ringing preventive circuit ofclaim 2, wherein the output stage of said voltage follower operationalbuffer amplifier includes a push-pull amplifier.
 5. The ringingpreventive circuit of claim 1, wherein said buffer amplifier comprisesan emitter follower.
 6. The ringing preventive circuit of claim 1,further comprising:a second comparator having a first input terminalconnected to said output terminal of said buffer amplifier, a secondinput terminal provided with a second reference voltage, and an outputterminal, for outputting a second control signal from said outputterminal in response to a result of comparison between voltages of saidfirst and second input terminals of said second comparator, and a secondcurrent supply circuit having a first terminal connected to a secondpower source for supplying a second voltage, a second terminal connectedto said input terminal of said buffer amplifier, and a control signalinput terminal connected to said output terminal of said bufferamplifier, for controlling conduction and non-conduction of a currentflowing from said first terminal to said second terminal as a functionof said second control signal.
 7. The ringing preventive circuit ofclaim 6, wherein a circuit composition of said first current supplycircuit and the circuit composition of said second current supplycircuit are identical.